Please see below an update from GWR:
"We have a new timetable starting on Sunday 13 December, with both weekend and weekday changes. Online timetable journey planners have been updated, and you can download copies of specific timetable booklets from our website: https://www.gwr.com/plan-journey/train-times.
As with all current journeys, we are asking customers to please check their journey before travel on www.gwr.com and book in advance, reserving a seat, and for advice about travelling with confidence there is more information on https://www.gwr.com/safety. This also applies to travel over the Christmas period."