Firstly thanks for the advice.
I removed the head as you said and the pin was well and truly stuck. After some gentle persusion with a hammer,WD40 and pliers I seemed to have released it. Don't know if totally but it is definitely has some movement now. It must be doing something because the...
Just recently hot water has become very cold to lukewarm. Remembered similar problem 2 years ago which was diagnosed as the motorised 2-port valve.
Tried manually switching valve into manual from auto and this seemed to cure the problem however it seems a little intermittent.
Replaced the...