Thanks chaps for the advice. I went into the local Plumb Center and
picked up an acutator for only £35 inc vat. Fitted it and now the CH
is working fine.

Only concern is that this is the 3rd actuator in 4 years. First one in
was duff and now I've replaced the second. Also when I contacted
another local plumbing merchant the person I spoke to said that their
relative was on their 4th actuator in 6 years! So are Myson products
duff? I noticed with the one I bought that there was no mention of any
guarantee.