I thought I would post further details and see if anyone can come with more suggestions.
On starting up it is very "lumpy" - for want of a better word - and smells very strongly of petrol. Until the gauge reaches normal running temperature there is next to no power under approx 2500rpm. It has to be revved quite hard.
As it it warms up, when slowing down the engine does return to a normal tickover speed slightly quicker but nothing like as quickly as it should. When warm it drives reasonably well but still has a little "lumpiness" when driving at 30mph in fourth.
I have changed the rotor arm and plugs and give the engine flush and oil change as a sticky tappet had been suggested. No change really. I have checked all the vacuum pipes I can find and did find one with a lsight split so that was changed too.
I am starting to suspect a sensor but don't which one to go for!