A bit more progress last night! Effectively what is happening is that the boiler is cutting out before reaching flow rate temp (as if the thermostat is cutting in) and in my limited knowledge I see that it should work like say a kettle i.e keep going until temperature reached.
So last night and in the traditional way I would give the control box a small "wack" each time the boiler cut out and lo and behold it would come back on again!! Using this method I managed to get the flow temp up to about 75 degrees. A bit low compared to spec but 20 degrees higher than i was getting!! So I suggest that some electicrical connection (of sorts) in the control area is problem, any ideas. (please note I have to wack it each time to get it going, one wack is not a long term fix!