Running on 3?

Take the rocker cover off & check the valves on number 4 are actually opening.

first thing I would try is to spray carb cleaner around the inlet manifold (with it running) any change in speed will show there is an air leak there.

If you can take one sprak plug lead off at a time and the engine doesn't sound any different to what it does with 4 connected - then one of the leads is broken. If I take a lead off the engine sounds completely different. The piston probably has a hole in it. I doubt the engine will run for much longer. One of my cars went like that after 100,000miles. At first it lost a bit of power, then sounded rough, then like it was on 3 cylinders and finally it packed up and went to car heaven.

"Paul P" gurgled happily, sounding much like they were saying:

Not necessarily. There's a myriad of reasons why one pot may not be firing...

...and that's about the least likely...

Well thanks for the suggestions. Was going to take the rocker off but first gave Mr C's suggestion a try and glad I did. Spraying around the inlet made no difference but then I discovered a slight increase when spraying around the servo hose. There is a small hose that connects the end of the servo to the inlet and is covered by a heat shielding material. Took off the heat shield to find the hose had perished and split. New hose fitted and good as new. Cheers all.

Sounds pretty extreme!?

Could just be a dodgy plug...

What, when the misfiring cylinder compression tests at 140psi?

I don't think so...

Chris