Thanks for your reply Tony
when the boiler was serviced and jet changed then gizmoes abounded and all sorts of readings taken
the system, valve and pump about 12yoa and the boiler about 8yoa - up 'til first problem previously been running perfect
read some other posts and earlier had...
have a boulter camray compact 50/70 oil based central heating system
water via f/e header tank
got an electric valve and the pump is grundfoss 15/60
about 3 months ago lots of banging whilst boiler running/heating, sounded like kettling at the boiler
rads and water get hot no problem...