[Gregtenerife]

Thanks for the answers guys. It has the 04 cap at the moment, I'll check for an upgrade. The coolant tank is fine, I've had the carpets out and there is no sign of a leak and no coolant smell. I still don't understand how the system holds and pressure if the bleed valve opens on its own!
"and i know you said you never touched the cap prior to refilling, but it doesn't mean the PO didn't put it on wrong. they are very easy to put on crooked (i did) and lose coolant (i did) as a result. at 1500 miles a year, it's possible the car wasn't drive to the point that would cause a loose cap to cause dumping yet."
Possibly but the 3000Km thrashing I gave it a couple of weeks ago should of highlighted any faults I'd of thought.