Don't bother with the dump valve mate...unless you really like the noise. You've already experienced one problem with them, but maybe it was the wrong type or knackered.

Although to be fair, I ran with a bailey d/v on my S2 for years and never had a problem with it. If you do want one, get a bailey item.
However, I sold it when I got the S1 as I didn't want to fit it and started getting slight chatter anyway. Much better noise imo.

You got any pics of your car??