Thanks again.
One further conundrum which I have been unable to solve (as yet).
No exact date this time, other than a Saturday in the summer months of 1980 (so no pressure you fine amateur detectives

), so could be last days of the 1979 GBTT or more likely in to the 1980 GBTT.
Something in back of my distant memory is saying the train was a Fishguard Harbour service, starting Paddington and calling at Reading. So am guessing likely 0926/27 ex Paddington, also due to pick up only at Cardiff (11u49), or at a push the following 0938 Paddington to (Tenby - 1979 TT/Pembroke Dock - 1980 TT) which had an open stop at Cardiff 10 mins behind the Fishguard train.
On the occasion to which I refer it was worked by an unknown (not recoded) Brush 4, the said loco expiring West of Swindon, or just after passing Wootton Bassett Junction as best as I remember. How long we had to await a rescue I do not recall, but this did come in the form of 56037, which took the train forward to Cardiff, where 'one' alighted.
Presume both locos were detached (the 56 is noted as being on Canton during those few days in the RCTS Observer mags), but have no idea if the train went forward, or was terminted?
Here is the formation just after arrival at Cardiff. Thanks again if anyone can shed any light?