After a bit of investigation and consultation - thanks SimonAl, Dunk and Phil Crouch - I hve just had the viscous coupling replaced.
I appears to have developed a mind of its own - from cold it started engaging correctly but as heat built up it went on strike then decided to cut in at the most inappropriate moments and lock on.
This was then causing the electric fan to work overtime and resulted in it all getting somewhat confused.
Anway, the final diagnosis was a "failing" (not "failed") viscous coupling along with a rather worn tensioner system.
All changed now and a couple of long runs planned in the very near future to see how things have improved. Not sure how much of an impact this'll hae on top speed (as I will be on public roads in the UK and so am unable to assess this but I would expect to see improvements in economy (which have been dropping recently for no obvious reason).
Will report back on this after the weekend