Quick update, the car has been getting gradually worse and difficult to drive, so pulled my finger out and changed the plugs this morning, first impressions where that the plugs looked in good shape, nowhere near as black as they were when you last took them out Paul.
I changed them anyway for a new set of GM plugs and have taken the car for a quick 5 mile drive, what do you know..... Completely sorted! No misfire or hesitation under any load of throttle or revs!
Thinking about it I did once drop one of the old plugs on the driveway, perhaps i damaged it.