oh and if you really want an old cav, check the auctions i have seen some strugle to get £50, although not much more than a real banger lol 
Damn your auctions are cheap!
They don't seam to go that cheap down here. 
Auctions up north do seem cheaper.
Guy i know swears by an auction house in Manchester....and lives in Swindon
He bought 26 cars the other week up there.....mostly astra's, the odd rover 75, some 406 pugs, all diesels and between 3/4 years old. Dont think he paid more then 2.5k each for any of them!
I go with the advice given by others....make sure its got a fairly long mot on it.
If your limit is £500, stick to it.......otherwise before you know it you've ended up paying £600.
Remember there will be a buyer premium, its usually bout 5% but there may a min amount so check it out first and adjust you max limit accordingly.
I would pay a visit to the auction first, to suss out how things work and you get a feel for the place, check out a few cars, see what price they fetch, then you'll see how much they usually fetch.
You dont want to go paying £500 on a motor, that usually only fetches £250.
On the other hand you dont want to go paying £100 for a motor that normally fetches £250....theres probably summat wrong with it that you havent spotted
Stand/sit where the auctioneer can easily see you, then what i tend to do is have a bid about 1/2 way through, so the auctioneer knows you interested, then wait until i think no one else is going to bid and if it stays under my limit one or two bids more and hey you just bought a motor

HTH