Grahame were you able to ask,when problems occur Chippenham/Bath Bristol,could the IET▸ from London be turned round at Chippenham and start/terminate at Chippenham.
Question not asked - however, that would be an unusual scenario. Technically possible I believe though you would have either two long single tracks from Wootton Bassett OR need to run the train beyond Thingley East Junction and cross over there - may be an issue with 5+5 as driver could not walk through.
Chippenham to Bath via Box is typically pretty reliable (I hope that is not famous last words!) and trains can be turned at Bradfiord Junction, Bathampton Junction (not sure about IETs) and at Bath Spa Station. Much more useful and used at Chippenham is turning trains arriving from the west.