IO540 - sorry it's taken a week to post, but you were right!! From an email sent to club members last Weds:
Sorry folks but I have today grounded the aircraft following an afternoon at Perth trying to produce a fix for a problem xxx encountered yesterday. The oil temperature guage has ceased to function and there is no way of monitoring engine temp. I have managed to source one which we can borrow until ours is fixed - the new one is yes you have guessed a Fuji part and as readily available as rocking horse jobbies. It is a borden tube type so they apparently are repairable and recalibrated- more on this as it unfolds.
So I was blaming the ambient temperature, when it appears that all the time, there was a gauge problem!!
Moral of The Story: Never Assume, CHECK!!
Thanks, guys!!