There have been no answers to the query about what 5573 was doing at Brent so I will offer my two pennyworth.
5573 was at Newton Abbot Shed from December 1957 to December 1962 when it was transferred away to Whitland. I photographed the loco on the Kingsbridge Branch on Saturday 1 July 1961, and I have seen a picture of it on the Branch the previous Saturday. A month later I photographed it at Wadebridge and Bodmin General but it was not formally transferred to St. Blazey so perhaps it was on loan.
The previous summers would have been similar but lets assume that this is a Saturday in June or July 1961. The train is the 10.55am Saturdays Only Kingsbridge to Paddington. The train would have arrived in its normal platform in Brent at 11.30am. 5573 would then have propelled its coaches out towards Newton Abbot over the crossover and onto the up main. It would then have worked its train westwards through the up platform out under the road bridge from which the photo was taken and into the up loop. The 11.15am Plymouth to Paddington arrived in the up platform and 5573 propelled its coaches out of the loop and on to the back of the Paddington train. Departure from Brent was scheduled at 11.45am and arrival at Paddington at 4.18pm.
Hope this helps.
Mike Many thanks indeed Mike
N.B. Another suggestion, which Mike thinks more plausible than his own - this comes from Charles Fennamore and is added to the Plymouth - Exeter section.