The race in Sept is a support race for the 6 hour, I believe. Main reason for it getting great reviews is that the quality of the other cars racing over the weekend is even better. Historic F1's and Can Am cars mentioned.
I run a Clubman Race head from MED, so it runs their double springs. I worked out that with practice sessions I've done over 7.5 hours of racing so far on it, and maybe 2000 road miles. Their view is that the springs are likely past their best after that. For £50 and a head gasket set it's worth trying, plus I get to check inside the engine top end. Before Donington I had to keep an eye on the rev counter to make sure I changed up not much over 7000. But last weekend it was going flat at just over 6500. No other symptoms. The only other thing I can think of is if the cam has started to wear. But I took a lot of care running it in so no good reason for that.
I'm hoping to do the 30 min Bentley Club Silverstone race on 14th Aug as it's better value (£/min) that the regular 750 MC race at the end of Aug, and most of our regulars are going to be there.