Pretty simple solution would be for FGW▸ to start their late Sunday afternoon every 30 minutes frequency between Bristol to London earlier so there is a 1600 service to Pad between the 1530 and the 1630...which is what they are doing from the start of the winter timetable on December 12th.
Which doesn't appear in the new Bristol to London timetable. Is this not happening now and that the last edition of
Modern Railways was incorrect to mention this in their Winter timetable review? I don't have a copy anymore to refer to remind myself of what was said.