Great Western Coffee Shop

I'm a few days late posting this, but does anyone know the details of the problem experienced somewhere around Chipping Sodbury (possibly in the tunnel?) on the afternoon of 5 Jan?

I was on the 1545 Paddington - Swansea service, and the train manager warned us after Swindon to expect a delay because of "lineside equipment failure" or "signalling problems" (he used both phrases at different points) in the Chipping Sodbury area. Apparently trains before us had been diverted via Bath Spa and Filton Bank, with us being the first service back on the booked route. In the end the delay turned out to be fairly brief - we came to a stand for a few minutes while I assume the driver had a chat with the signaller, then set off and were back up to line speed shortly afterwards. About 10 down at Bristol Parkway, although that was up to 15 after Newport thanks to some pathetically lax station work where the staff appeared simply to watch a steady trickle of people getting on from the new footbridge rather than making any attempt to get the train on its way (there was another service to Cardiff right up our backside by this point, so it's not as if any of the runners would have been inconvenienced!). Almost right-time by Swansea though.

Any ideas what the root cause of the problem was? I'm making an educated guess at a track circuit failure...