Did the tour from Paddington to Stratford on Avon 22/03/92 first pick up was Gerrards Cross, which was where it was discovered we had left the guard at Paddington. We left from one of the more highly curved platforms and I think he had to move a long way from the train to check all the doors. A Guard (not the original one I believe) eventually had to walk up from the signal behind the train before we could proceed.
While not a railtour you remind me of my favourite Irish Rail story. Some time in the 70's or 80's a guard at Dublin Heuston waved off his train as normal and jumped into the guards van only to discover that this was a SPARE van that had been shunted up against the buffers Apparently a taxi was hurriedly summoned to take him out to the first stop west - Newbridge or something like that where the train was held awaiting his arrival.