Ok, well with three leaks sprung and sorted I'm STILL loosing coolant!
Firstly the under floor pipes where seeping so new stainless steel ones were fitted, then under a pressure test the radiator showed a minor leak. Although as it was only very minor it was left for the time being, that is until it decided to split completely. A new rad was then fitted. Just after having the SS pipes fitted the inlet manifold gasket decided to give up the ghost so that was also replaced.
Now I've still got a VERY minor leak but just can not find it. I've replaced the expansion tank cap with a genuine one, topped it up yesterday and today the coolant is down by approx 1mm. I've checked for leaks under the F and there is nothing visible. I've checked the oil and everything is fine there also and the coolant is a nice red/pink colour.
I just don't know how I'm loosing it