Also I'm planning on going to Avebury for the occasion - its one of my favourites - any tips on where to park, when best to get there etc...
Parking anywhere in Avebury will be difficult if not impossible. Having said that you might still be able to get a B&B in the area and, if you can, you should get a parking place to go with it; you'd then have to walk or take a bus into Avebury itself. Failing that you could park outside Avebury but you'd still be looking at a 20 or 30 minute walk to the circle/village itself (and you should be aware that the area is notorious for car break-ins at all times of the year).
Devizes, Marlborough and Swindon all have good and affordable accommodation - you might want to consider booking into somewhere there and then bussing or taxiing into Avebury - not ideal but a lot less hassle.
