The answer was a Castle Class Class 2 + 3 HST and not a Castle Class 2 + 4 HST which are the regular formations.
Many Thanks Roger Winnen
Following experiments with single car Railbuses, featuring bodies produced with Leyland National bus components, British Rail Engineering Ltd, Derby built a 2-car railbus in 1981. This unit 140 001 featured a higher floor, high backed seats and the Leyland National bodywork, but with a rather austere front end, providing for a through corridor connection. This unit was extensively trialled around the country and I was very fortunate to come across it, quite by chance in August 1984, waiting in the loop at Dawlish Warren.
The experience gained with this unit went on to develop the later, production fleets of Pacer units, but without the corridor connections at the cab ends. This prototype unit remained in BR's fleet until 1994, at Neville Hill, Leeds, latterly as a parts donor for the later Pacers in service. I understand that this rare unit has been preserved at the Keith and Dufftown Railway in Scotland.
Very best wishes,