...the thing is... (and reasons why I'll persevere with the dalek cap)
1. when i had it checked out by the mechanic, he said the head looked fine and the seals were good... also, i can hear the air hissing out of the dalek when the engine is warm...
2. when i first looked at the system - before realising the the dalek was blocked, the header tank was full to the brim and under pressure - clearly it had been running with the dalek blocked and was therefore under more pressure than it should have been - which is maybe not good for seals etc... but if the head was cracked or there was a leak - it wouldn't have built up that pressure, right?
thanks again for thoughts and advice.
Any other way i could check the system in order to diagnose if I'm right that it's just the leaking dalek cap?
the woman that owns it wants a decision this weekend

may need to make my mind up before i have a chance to find out for sure!

A mind is like a parachute... only works when it's open.