Thanks for the info Chris! I understand what you are saying, last night after posting here, it played up again
It was a warmish night, and it struggled a little, 40% to get to -11! Eventually got down to -33 or so, but at 100% I manually raised the set point to back off the power, and it stabilised at -34, with 70% power.
Maybe I will try gradually dropping the temperature in 5 degree intervals?
Erik