Thank you Eric and Rossocorsa that's a great help.
That gives me enough information to clean out and replace the Coolant. I will get hold of a Haynes Workshop Manual as a matter of course.
( The Jam thermometer is a useful tip
..In anycase, I'll give it a go, Your 'Oil in the Header' comment was of slight concern however as I found some fine scaly deposits
sitting on the surface of the liquid on inspecting the contents of the Header receptacle. ( see below ) hopefully it's some other kind of residue. ( fingers crossed )
Rossocorsa thank you for your offer to help out with details for later models, it's mainly the Digiplex, digital advance ignition system that I'd like to get to know better because I understand
that they sometimes can be troublesome.
Thanks again, Andrew