Thermostat is most likely cause and every chance it's 19yrs old - ordered one up and installed today, problem solved.
The old one is freakin Fubared , somehow it's bent the rising pin and it jammed wide open. Hooked in the analogue meter too after doing some testing in the flat with it and it works a treat.
At tick over it got up to 95'c no problem and while driving about was hovering around 80+ ( tonight is about 3'c) so well happy with the result
Word of advice, if you haven't got a slimline spanner set - I'm talking about the thickness around the ring spanner end get an 8mm one now! Some bright spark decided to put the thermostat bolt directly in front of a frame cross beam :smt013 Trying to use the open end of the spanner just started to round the fugger
So after much cursing at the genius who came up with this I got medieval on the thermostat housing (I've got loads of spares) with my chisel and hammer
It now fits my spanner 
The other option was to dig the angle grinder out and modify the spanner but the chisel and hammer were just lying there 
Best solution is to change them when you have the engine out of the frame so the J2 is getting a new one thrown in it just now - I did test it and seems to operate ok but aint going through that again
But, standard gauge is now working , the new analogue one is plumbed in and everything is tickety boo again - result !
BTW, the new thermostat I put in has been redesigned and looks to have no visible pin so maybe them bending had been a problem recognised early on
