Well, just to finish this thread off - I installed the "gold plated" Westerbeke oil pressure sender unit and Bingo -a steady reading on the guage - yahoo.
Interestingly, the old one was quite heavy, suggesting it was full of oil. I intend to cut the old one to see how it works - perhaps it has some sort of bladder arrangement in there that expands with the oil pressure acting on a rheostat (or something) and that burst.
Thanks for your comments
IG 36 Europa