I'm just seen that extra services between Cheltenham and London will be 9 or 10 carriages. Can anyone tell me what services they will be?

Check before you travel as Great Western Railway starts new winter timetable from this Sunday | GWR News
Direct services between Bristol and Oxford will continue this December, and extra carriages are being put on some more Sunday services, but customers are warned to check their regular train times as GWR changes its timetable.
Recognising changes in customer demand, the train operator is also to increase the number of carriages on some early Sunday morning trains on key routes. Nine or ten carriage trains will replace five carriages trains, effectively doubling the number of seats available on some services from Cheltenham to London Paddington and from Great Malvern to London Paddington.