The new thermosensor or CTS arrived today and fitted it - hey presto it works as it should.
What is a little surprising is the fact that the first 2 resistance readings showed an error but the others were a constant decrease as the CTS was heated up. Also the fact that it worked fine when heated with a lighter when not in the engine. So I have come to the conclusion that it is not earthed in situ as all other systems worked fine and apparently share common earths.
I think as Hugh said - it was probably leaking to earth with an internal fault even though I couldn't measure any leak under test.
Thanks to all the replies especially Hugh and hope this will be useful for someone else when they have a similar fault, it was time consuming and irritating but a satisfying outcome.
