Parking at the ground is for permit holders only. Mitch Ford informs me; 'Parking around Ashton Gate has been severely hindered recently in that many streets now have double yellow lines. There is the Bedminster Cricket club on Clanage Road (A369), that offers parking at a cost of £5 per car. It is then around a 5-10 minute walk to the stadium'. Otherwise it is a case of finding some street parking.
There's a cricket club not far away. If oyu come off jnuction 19, turn left at the roundabout and just keep going you'll see it on your left. Cheap and not a long walk from the ground. Thats where we normally park. Plus if you get there early enough it'll be empty.
When I went 2 seasons ago I managed to park in the Wickes car park right by the ground. This was an evening match though so might not be so easy on Bank Holiday!