Get on train as passenger to minehead. Once there asks to uncouple the loco from carriages, do that. Nothing happens, no next objective, nothing.
The scenario only works if you uncouple the carriages AFTER the doors close. The instruction to uncouple appears too early.
I don't think it's that long. It's a normal length unloading cycle, so if you get off the train and wait for a minute or two, the doors should be closed.
Bit of a bump... Just had this scenario come up in Quick Play. Found the Class 33 is suffering severe brake drag whether on its own or after recoupling to the stock. All brakes released and switches in correct position, no handbrake applied at either end but you need to gun the throttle to about 50% just to keep moving.