Not a lot of people know this.... :-)
Just finally diagnosed a problem with my wife's Rover 220. Its not well known and took ages to Suss, so here is the news.
I found the problem after searching on Google and finding a picture of an identical set of Honda plugs which had caused the same problem.
The symptoms are occasional misfires, jerkiness, mostly when cold and at low revs pulling under load. This gets worse and then includes poor cold starting.
The plugs looked weird, gapped OK and not corroded at all, but with a uniform rust coloured deposit all over them, I know all the usual symptoms having been bought up on carburetted oil burners. Still nostalgic for that lightbrown 14:1 mix on the twin SUs.
Turns out the colouration is a deposit left by an Octane improver called Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT), used by some petrol companies but not others, banned in many US States, just re authorised in Australia, and clearly used by Tesco, my wife's petrol supplier of choice.
See pics for a better idea. (Yes the No 4 cylinder has the classic Rover T-Series engine head gasket failure ongoing).
New plugs totally cured this long running problem. I have not yet tried cleaning the old ones. I suspect with my anti environmental point and squirt between corners and as fast as you can get away with on Motorways "enjoy the HPT while you still can" driving I'm OK with my 9000 Aero, but if you are a gentle caravan towing driver of a regular injection non turbo you may experience this - particularly with a caravan - serves you right :-)