Whether there are any advances on sale for Bristol, let alone those cheap ones, is another question.
There are indeed cheap fares, I've just booked one for December.
Had to do a bit of work with the booking engine (Red Spotted Hanky, redeeming my Tesco vouchers) to get it though. It offered the following itinerary on the 08:25 from Ash at £57.00 advance:
Ash - Reading 08:25-09:00
Reading - Bristol Parkway 09:11-10:04
Bristol Parkway - Bristol Temple Meads 10:12-10:24
But, specifying "avoid Bristol Parkway", I managed to get the following at £11.50:
Ash - Reading 08:25-09:00
Reading - Bristol Temple Meads 09:29-10:43
I'm happy to delay myself by 19 minutes to save £45.50, and also reduce the number of changes. Presumably the cheapest fare is still available because most people don't know to add an avoid to the search.