Looks like it was a really really bad day for GWR generally. The delays throughout the whole of the Bristol area have been staggering, with some trains running at least 80 mins late.
My train this morning was about 20 late and had no air con in any of the carriages (was a refurbished 158), and my train back was 30 mins late. Looks like in addition to the signalling problems (there were some towards Portsmouth in the morning too), there also look to have been train faults all over the place, in turn leading to crew shortages!