Ive been to Silverstone for 4 races over the last 6 years. We used to Sit at Club when it was still just a bank and it was great, but there is almost nowhere left for general admission any more.
Sat near Duke in Luffield B this year and its a good spot. Woodcote A is the next grandstand along and is just as good. I'd go for eaither of those.
If your going for 3 days a grandstand gets you a reserved seat on Sunday, but a roaming pass Friday and Saturday so you can go anywhere. Spend Friday walking the whole track, sit everywhere you can, and get a feel for it. Then on Saturday pick out some good sports to sit for the support races and qualifying.
There is pleanty to do around the track, pleanty going on, pleanty of places to spend money, but also the track is always busy with a number of support race series going on too (I'd definately recomend watching the GP2 races).
I havent camped for the F1, weve drievn up previous years but I wouldnt recomend it (although from Plymouth its not really possible for you). This year though we stayed in a Travelodge about 20 miles away near Milton Keynes. Was the closest place that wasnt charging stupid money, and its nice to have a proper bed and a shower. Just get car parking and drive to the circuit. The whole dual carridgeway to the track is made 1 way over the weekend so traffic isnt that bad. Having spent 3 days at Spa in a Motorhome, id definately recomend a Travelodge if you fancy sleeping, but camp if you want to jump into the atmosphere of it all, but dont mind Finnish Thrash Metal played all night from a neaighbouring tent
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Bottom line, its a great weekend and you will love it. The atmosphere is fantastic (especially if the Brits do well). Pleanty of us on here have been if you have any specific questions.