illegal. WOF time...
Join Date: Mar 2012
Location: New Zealand
It sounds like the thermostat is stuck open for sure - whether that is the cause of the misfire or the overfuelling isn't certain, but it could be. It won't be helping. Your idea of blanking off the radiator should be a good diagnostic (unless the ambient air is REALLY cold).
If it is the problem, the part is not too expensive and the method of changing it is very easy so you could have a go yourself. I paid $NZ120 so that's about 60 pounds for the part, plus some coolant and liquid gasket. If you make plenty of cups of tea, the mechanic should do it for free...