For the record - from Guy Vincent.
Roger's photo of the 37-hauled relief train arriving at Penzance in August 1985 warrants investigation and comment.  Back in 84-85 I spent many enjoyable and sometimes frustrating Saturdays on 'Devon Explorer' day rover tickets riding around on as many different locos as I could get.   The high summer 1V32 1030 York-Plymouth or Penzance relief was much anticipated while it was 'in season' with many enthusiasts turning out to cover it. I think it ran as a relief to the 1V85 0923 Newcastle-Penzance.   Here is a list of recorded workings for you:

Saturday  27th July 1985  37024 with 6 Mk 1 coaches. 1V32. Allocated to Tinsley, No Heat, Dual-Heat stock.  Haulage Exeter St Davids to Plymouth, depart Exeter 1613 (sch'd 1606) arrive Plymouth 1718 (sch'd 1716).

Saturday 03rd August 1985 47135 with 6 Mk 1 coaches. 1V32. Allocated to Crewe Diesel. Boiler Isolated, Dual-Heat stock.   Haulage Newton Abbot to Plymouth, depart Newton A 1634 (sch'd 1635), arrive Plymouth 1718 (sch'd 1716). 

Saturday 10th August 1985 47254 with 10 Mk 1 coaches. 1V32. Allox Bristol Bath Road No Boiler Dual-Heat stock. Haulage Taunton to Exeter St Davids. Depart Taunton 1551 (sch'd 1535) arrive Exeter 1619 (sch'd 1606).    Note: This is the date Roger believes his photo was taken at Penzance.  I found out online that 37113 worked the train from York to Bristol Temple Meads where the 47 replaced it. This probably explains the late running beyond Bristol. There is a photo of this train at Bristol with 37113 about to be detached, it has nose-end gangway doors.

Saturday 17th August 1985 37058 with 9 Mk 1 coaches. 1V32. Allox Gateshead Non-boiled Twin Fuel Tanks Dual-Heat coaches. Haulage Taunton-Plymouth. Depart Taunton 1544 (sch'd 1532) arrive Plymouth 1753 (sch'd 1715).  As I recall we lost time between Totnes and Hemerdon due to sheep on the line, somewhere around Wrangaton or Marley Tunnel I think it was.   Note: This is I believe the train Roger recorded at Penzance.   It has 9 coaches and the loco is quite clean with a plain panel in place of the gangway doors.    It's last overhaul had been completed in May 1985 so the condition seems to reflect that.

Saturday 24th August  No Record

Saturday 31st August 1985  45003 with 8 Mk 1 coaches. 1V32 Allox Toton Boiler isolated Dual Heat coaches. Haulage Taunton-Plymouth. Dep Taunton 1535 (sch'd 1534), arrive Plymouth 1718 (sch'd 1714).

No further recorded workings and as the peak summer period was over I doubt the train ran again.

See websites 'The Bashing Years' and Class 37 Locomotive Data Site for more info.

And also from Clive Smith

Some superb follow up pics from Roger Winnen on the reliefs with 37s. I have been contacting my fellow enthusiasts from back then and we can confirm the mystery loco is 37058 and the date is in fact 17th August 1985. Hopefully we have stimulated a discussion on the subject of reliefs and maybe some more photos will be forthcoming. 
And:- The final piece in the jigsaw is provided from information on the Class 37 Loco Group website. On 10/08/85 37113 set off from York to Penzance on the relief but alas was replaced at Bristol TM by the much less exciting 47254.

I expect Roger will be very busy with the VSOE , the St Philips Marsh visit and is he intercepting the GB XI with 45212 and 31806 ? I am sure Guy Vincent will be out for that too !

I look forward to all the latest updates.
Regards  Clive

I thought that it might be of interest to CRS members and others to hear about GWR's intention to introduce tri-mode "Flex" trains being made available this year by Porterbrook.
Although we are unlikely to see these units in the west, the concept is an interesting one and might have wider applications in the future.  The following is an extract from Porterbrook's description of the trains:
The Class 319 Flex concept is designed to create a bi-mode train by fitting two diesel powered alternators, one under each of the driving trailer cars. The diesel alternators provide power to the existing traction and auxiliary equipment to allow the EMU to operate without an overhead or 3rd rail supply. The systems will provide power through the train’s DC bus, avoiding any significant changes to the existing equipment and creating a unit capable of operating from a number of different power sources whilst maintaining its full capabilities on electrified routes.
The plan is to convert some of the fleet of class 319 units previously in service on Thameslink where their 25kv ac overhead and 750 volt dc third-rail capability was the key to operating services across London from 25kv networks north of the Thames and the Southern third-rail network to the south.  In a press release dated 19 April, GWR has announced their acquisition of nineteen of the Flex units and the significance of this is considerable, for example, being able to operate between Reading and Gatwick Airport where most of the route has third-rail electrification but the section from Wokingham to Ash is not electrified. At the same time, these units will be able to use the 25kv system out of Paddington - but could, if required, operate along the Thames valley branches which, under present plans, are to remain unelectrified.  GWR's press release describes the company's plans:
Great Western Railway and Porterbrook to introduce the UK’s first tri-mode train

The first fleet of trains able to run on overhead and third-rail electric lines, as well as under their own power, will be introduced onto London and Thames Valley services next year following a deal between Porterbrook Leasing and Great Western Railway.

The 19 Class 769 Flex trains begin to be introduced onto the Great Western Railway network from Spring 2019. This will release a similar number of popular Turbo trains to add capacity in the Bristol area, offering the potential for launching new routes through the city.

This is the latest step in Great Western Railway’s drive to provide more seats across its network and builds on the 8,000 additional seats delivered into and out of London earlier this year.

This innovative fleet of tri-mode trains will operate in four-carriage sets which will be refurbished inside and out, with free WiFi and power at each seat, air cooling, more space for luggage, and new seat covers. Its modern two-engine design will offer a quieter and cleaner experience for customers.

Initially, the fleet will support the introduction of refreshed trains on Heathrow Express services, but will be predominantly be used on routes between Reading and Gatwick, and Reading and Oxford, where the train’s tri-mode can be used to its fullest. However, the tri-mode nature of the train will give GWR maximum flexibility to use them in other areas of the network should they be required. 

Final details of exactly how and where the new fleet will be used are currently being drawn up, and expected to be finalised later this year.

GWR Managing Director, Mark Hopwood said: “We are already delivering the biggest fleet upgrade in a generation across the Great Western network, but today’s news shows we have not stopped looking at ways to improve our service for customers by adding extra capacity to our network.”

Mary Grant, CEO of Porterbrook, said: “I am delighted that Great Western Railway have agreed to add our innovative tri-mode trains to their fleet.
These trains will offer GWR greater operational flexibility with reduced operating costs; at the same time their conversion and upgrade will see skilled engineering jobs secured in the UK supply chain.”
How does this affect us?
I think the somewhat hazy connection between the tri-mode announcement and "our area" is that this now looks like the only way to free up more class 165/166 units to cascade westwards following the decision not to electrify the Thames Valley branches and the delay in electrifying from Didcot to Oxford.  The 16x units are destined for Bristol and, in turn, class 158s would come further west. 
With many thanks to Derek Buttivant for providing us with this important news.