A few months ago the head of our drayton trv (not sure if it is a 3 or 4 version) was knocked off & broken so we had the head unit replaced. At first it was misaligned so it wasn't pushing the pin down at all (heat full blast from that radiator). Suddenly this has started happening again. We've been doing some decoratoring in that room so my have 'touched' the radiator/valve but certainly no major jolts. I'm going to have a go at taking the head off tonight and aligning it again to see if that works. But otherwise any suggestions as to why this is happening again? Do we need to replace the whole valve unit in case the pin is not working?