Great Western Coffee Shop

What's the maximum length of train that can be turned back at Warminster (from the Westbury direction)?  Any limitations about the type of train?  Clearly, there's no run round, but what about a train that has no walkthrough facilities such as a 150/1 plus a 153?

In theory twelve cars, although for practical purposes five cars (as that is the max platform length).

A sticky point is that there's no authorised walking route next to the crossover on the Down Salisbury. So, without a walking route the driver has to stay on the train, which as you suggest would stop a 150/1 operating in multiple with anything else (unless a 2nd driver is provided). However, creating an authorised walking route is not difficult to do.

I've taken a 4 car rake down to Warminster to turn back.  This involved a Class 150/2 and a Class 150/1.

I can concur that changing ends on non corridor stock at Warminster is challenging.  We ended up asking for a block on the adjacent line as the shrubby was just too overgrown in the cess to do it there.