Sundays on Great Western has quite a cut, hopefully back to what can be reliably delivered on a week in week out basis, details taken from the pdf timetables on their website:
07.37 Reading - Basingstoke and 08.06 Basingstoke - Reading deleted.
Bristol Temple Meads to London services drop to hourly through the day, departing Bristol at 07.45, 09.00 then hourly to 14.00, an additional working at 14.30, then 15.00, hourly to 18.00, then every 90 minutes at 19.30, 21.00 and 22.28.
Extensions south west of Bristol are vastly reduced with Bridgwater and Highbridge losing their Sunday direct trains to London, services from Weston remain at 08.29 (09.00 Bristol), 11.31 (12.00 Bristol), 13.31 (14.00 Bristol) then a gap to 22.03 (22.28 Bristol).
From Paddington departures are 07.59, 09.00 (extends to Weston), 10.30 (extends to Weston), 11.30 then hourly to 17.30, then 18.30 (extends to Weston), 19.30, 20.30 (extends to Weston), 21.30, 22.15, 22.45 and 23.33.
Through trains between Cheltenham and London are further reduced from the existing 3 at 10.47, 15.00 and 17.00 to just a single working at 10.47, the other two workings terminating at Swindon.
There are additional 387 operated services in some hours between Paddington and Swindon.
Cuts on the B&H are the 14.36 Paddington - Exeter is deleted, 4 hour gap in service at Pewsey but Newbury, Westbury and Castle Cary added to the 15.03 Paddington - Penzance. The 18:36 Paddington - Plymouth terminates at Taunton with Dawlish and Teignmouth added to the 19.03 Paddington - Plymouth.
10.32 Paignton - Paddington deleted, stops added into the 08.10 Penzance - Paddington for Pewsey and Newbury and the 09.18 (re-timed to 09.09) Penzance - Paddington for Castle Cary and Westbury. The 19.15 Plymouth - Paddington is deleted.
The 17.50 Cardiff - Portsmouth terminates Westbury and the 22.06 Portsmouth - Westbury is deleted.