The official car park is card only, just changed on NYD, pay on the day but only £8 if 2+ people in the vehicle (£10 otherwise), not sure if you can pay in advance. There are various unofficial car parks which will take cash (£5 upwards) or you can risk a side street for nothing. Be careful not to park in a resident permit zone though. When I've used the official car park I usually aim to get there about 1 hr before kick off so can't say if there'll be space any later on, but the later you get there the longer it takes to get off after the end of the game. It's taken me as long as 90 minutes to start moving after some games because I was at the far end of the car park. I assume if you get there really early you'll be nearer the exits so away quicker. You should be okay to find space in an unofficial car park 45 minutes before kick off (probably less) and it is usually quicker away after the game but you'll need to walk a bit further. Even doing this I can get home in about 75mins so much better than the official car park.
