Hello Dave,
Thanks for the reply. I did do a defrost for a day. It is a nodefrost fridge-freezer so there wasn’t any visible build up of ice.
I haven’t got much else to try however so I have unplugged it again and will defrost it longer… I found a small access hatch that views the cooling coils behind the freezer from the inside of the the fridge and there was a very light frosting on the coils so I will try again and wait until they are totally defrosted but I think it is probably some other cause…