Ok, fairly sure it's head gasket now...
Life is good if just bimbling round but as you start caning it the poor thing will gradually push coolant out into the overflow. Then when it's cooled down all the water pushed out fits perfectly back into the cooling circuit - it's not loosing any coolant. Still goes like a train, oil fine and now steam out the exhaust - ever. I'm suspecting it's very early stages of a head gasket failure that only passes when the cylinder pressure is high enough. Nearly 40k miles and I suspect a reasonably hard life.
I'm hoping it's just the rear cylinder as I don't particularly want to drop the engine if at all possible!!
Would appreciate any thoughts :thumbup