What started off as a occasional tapping from the engine has now developed into a loud rattling from what sounds like the top end of the engine. The engine has developed a missfire as well. The noise goes at tickover and the missfire seems to clear too, both come back if you rev the car.

A compression test showed no problems, not that I was expecting it to.

Is this just the usual CVH tappet problem or something more serious as I've never known it to cause a missfire? The problem has developed over the last 50 miles to the stage that the car is undriveable now.

 

cam lobe could be worn to the point where its not opening the valves enough to let a the air/fuel mix in at low air speeds ie, tickover.

 

Above is very likely the reason. Worn cams cause missfire's and rattles just as you describe. Compression won't be effected, as the valves will still be shutting. I have worked on a lot of RST's with your problem, and it is nearly always down to the cam.