>>>"High load/low RPM misfire could well be failing coil packs. I suspect it's a semi-sequential ignition system with wasted spark set-up"

Hmmm....is that likely to be expensive? By coincidence, I've just started to think about buying a new car later this year, so I was hoping to keep the Rover going at minimal extra cost for a few more months yet :-)

>>>"Does the problem appear to get worse as the car warms up from cold?"

If anything, I would say the opposite. Initially at cold it can be quite bad, but it seems a little better once the engine has warmed up.

It came on very suddenly - I took the car out one morning last week and the problem started. I wondered at first if it might be just dirty petrol and/or some sort of fuel line blockage, but would not that affect the general standard of running all the time?

Another possible clue.......????

I'm just puzzled that it ran for about 160 miles after the coolant leak before the problem appeared.....

Checking around the engine for possible loose/damaged cables, pipes etc, I noticed that underneath and behind the throttle is a cone-shaped item (see picture at LINK), attached to the throttle by a very loose piston-like linkage - what is this please? Coming out the rear of it is a pipe which is quite a loose push-fit (no securing clip etc), which goes off to something near the front underneath the battery area, but no idea what :-( Is this likely to be a possible suspect ?

Hmmm.....since posting my last message I've been told that it is probably the vacuum advance/retard mechanism. However, I was also told that the pipe coming out of the back of it should be connected to the inlet manifold, when clearly it is not.

The linkage from the throttle into the 'cone' is very light-weight (plastic?) - see the reddish-brown bar running from front to back in the picture. Also, it feels as if it is not connected to anything inside the cone, just free to move about. When the accelerator is depressed this bar moves into the cone. If it were cruise control, so it not be capable of being pulled in by the cone mechanism in order to increase the throttle when necessary?