Thanks for the advice Dave - finally sorted the issue - the thermostat/temp switch in the radiator had failed.
Swapped it and now working fine.
A word of warning though I was sold a sensor labelled as 2010 EVO and assured it was identical to my 2013/14 model...
It wasn't, confirmed by BETA UK - the wires are a lot shorter on the 10-12 models than the 13/14 so had to rewire it.. works fine now but a bit of a pain...