This is just another example of GWR fleet not coping at weekends, there is a very similar thread on services via Bath to Weymouth and via Bath to Portsmouth.
I have recently visited Wells and Tetbury, both full of tourists, but both without stations these days, so appears there is more demand on the GWR patch at weekends than at first glance.
The current fleet formation is too clunky, 5car sets can’t cope, doubling them up for part of a route is time consuming, 9 car sets can’t really be spared.
Its rather a shame they didn’t grab some secondhand class 91 and mk4 sets (or even some old Norwich sets) to use on Paddington-Cardiff at weekends to release the hybrids for busy weekend services on rest of their network.
At the moment they seem to be paralysed and not know how to deal with their new weekend business, and seem to be acting as though it will go away, rather than proactivly sourcing stock to serve it.