potterton netaheat 10-16?

Boiler fires up ok bur front of case getting very hot and not shutting down boiler thermostat does not seem to be controlling. I replaced with new but does not make any difference. Case still getting hot and if i leave on the case gets very much hotter and boiler does not switch of unless roomstat control cuts out. I have pushed up the pump speed and left running withboiler switched at boiler switch to see if i need to clear a blockage, still no good! Any ideas would be appreciated.