I bought from these guys last time, and they seem to be the cheapest around now, still.
http://www.peterlloydrallying.co.uk/fire-extinguisher?product_id=552
I also need to learn to take my own advice. Just days after saying "Don't buy wheels" I now have 8 Oz Superturismos in my garage, and another 2 on the way.
For the ARDs, take it nice and easy, keep it on the track. The multiple choice "exam" is pretty much common sense - make sure you've watched the Go Racing DVD a few times, know your flag signals inside out (even the obscure ones). I hated it because when I took it my closest circuits didn't offer it so I had to go all the way to Mallory Park to about twenty minutes on track and a ten minute quiz. Once you've passed, make sure you renew every 3 years - you're eligible to renew without an ARDs test as long as you've held a license in the previous 3 years, whether you've raced or not.
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"Talking about badgers is life. Anything before or after is just waiting" Steve McBadgerTransversa aurigatio transcurro arvae.