I sorted the problem at last. the engine ran on cylinders 3 + 4 only for around 5 minutes, then decided to stop shaking too much on tick over. ran for @ 3 minutes at 2500 rpm, then tick over seemed a little better. took a chance and slowly drove down the road, ran but still shaking. put my boot down, crept up to @ 2000 rpm, let out a large grey / white cloud of exhaust smoke. then took off like a rocket ship. drove @ the block, tick over still a little bit shakey . went around the block a couple of times, now running perfectly fine, with no problems at all.
thank you for all the advice given by various members