Does it really block the line for 40 minutes to turn round a train at Chippenham?  If not, I make the following suggestion.

The draft timetable proposes an hourly service from Westbury to London as an extension of the Bedwyn service. These trains are going to be pretty empty till they reach Newbury, so why not collect some West Wilts passengers by starting the service at Chippenham and calling at Melksham and Trowbridge?

Trains arrive Westbury at 10 minutes to the hour and leave at 14 minutes past. If they fitted in a return trip to Chippenham they could contiue to London at 14 mins past the next hour.I calculate one extra train set would be needed but in return:

1)  There would be better loadings between Westbury and London.

2)  Chippenham, Melksham, Trowbridge & Westbury would get an hourly service each way (an improvement rather than a cut)

3)  Melksham & the county town of Trowbridge would get a direct service to London. And not a bad one. The trains run non stop from Reading.

More expenditure but better value for money!

I think the following pattern would fit the draft timetable, provided the train can double back from the Up platform at Chippenham.

Paddington     dep  08.18
Westbury       arr    09.50
                    dep  10.00
Chippenham   arr    10.35
                    dep  10.44
Westbury       arr    11.13
                    dep   11.14
London          arr     12.51