It's old ground but freshly dug, Nick.   The reason given has changed in the last couple of weeks - from "there isn't the capacity to turn both an hourly Bristol service AND a Westbury service", it has now become "It can only be done under special circumstances with a Network Rail representative present".   That answer raised more questions than it answered with me, but I didn't feel it would be a good uses of resources to follow through.  I think the real answer, as with so many timetableing issues, is "because it suits our operational convenience to [insert customer-unfriendly action here]"

A Really innovative approach, of course, would have been to serve Chippenham with a TROWBRIDGE - Melksham - Chippenham shuttle in connection with a Bristol - Bath - TROWRIDGE - Reading - London train.  That would have meant that First's major flow from Chippenham (London) would have been train all the way and cut right down on the buses needed ... it would have allowed them to do any works needed on the Thingley to Batheaston section at the same time without a further closure, and it would have given us a TransWilts train service.   Such innovative approaches seem lacking, though.