dave, i can't see any evidence it has actually 'blown' as such, the firerings are all clean and intact, as is the head around the ring area, just the fibre material has gone very brittle, and broken up near a couple of waterways..
It was pressurising the coolant and lifting the cap after 4 laps of Castle Combe in the summer, hence why i've taken it off to look.. I've been using it on the road since, with absolutely no problems until november when i put it away for the winter... I'm supposing it was showing the signs of going under heavy use, and I've got it off before it really gives up bigtime..