Hi Mr T, at 5-30, you should be okay for street parking near the ground but any later and you probably won't find anywhere. Most away fans head for the already mentioned Bedminster cricket club at BS3 2JX. Nice and easy in and out, a short walk to the stadium and very sociable. Just be careful that you park in the cricket club at a fiver and not in the next door field where you will be ripped off.
One word of advice, even if your sat nav tells you to go on the M4 and M32 into the city, DON'T! Stay on the M5 as the city centre is a total car park at the mo. If you're heading for the cricket club, use J19 (A369) which take you to the entrance or, alternatively use J18 (A4) which brings you in very close to Ashton Gate.
Have a safe trip.