Towcester will probably be a nightmare too, Id see if you could do Sixfields, not sure where it will be in Towcester.
When the Website finally loads it might say on there :)
Or use Park and Ride, dont think you have to buy these in Advance!
Coming down the M6 you will be able to use to Park and Ride in Northampton Sixfields Stadium, at Junction 15A of the M1
http://www.silverstone.co.uk/popup/events/Spectator-Getting-There-2012-BGP/