I agree with Mattstorm. It seems that the only way to resolve these issues is to get rid of the car. That's what I did and I don't regret it. The two dealerships I went to gave all sorts of bullshit responses to the problems I reported, and these weren't just relating to the misfire/judder as I reported in my earlier post.
I thought my misfire was a fuelling problem. It just felt like the engine wasn't getting enough fuel and this problem would happen in various situations. For example I'd be driving along at 30 mph in 4th gear with the revs at approx 1700 rpm when the misfire happened and the car would judder. I noticed that when the misfire occurred under these conditions the needle in the fuel economy gauge (at the bottom of the rev counter) would swing rapidly from the 35 - 50 mpg mark towards the 15 mpg side (or whatever the lowest figure is - I can't remember), as if I'd floored the throttle, when in fact I hadn't moved it a millimetre.
At other times I'd be accelerating up through the gears and between 1500 and 2000 rpm the misfire/judder would occur and there'd be no power. A couple of seconds later the engine would respond and the power would come in forcing me to back off the throttle. If you've ever ridden a motorbike which has run out of fuel in the main tank and when you switch to the reserve fuel the power comes back in you'll know what I mean.
Occasionally the judder would occur while acclerating through the 1500 - 2000 rpm range but there would be no power loss.
Neither of the dealers I took the car to could find any reason for this behaviour as they said there were no fault codes and apart from the ineffective advice to run the car at high speed to clear the contaminants out of the catalytic converter they could not do anything to resolve these faults.
When this problem started to appear it was random and inconsistent however towards the end of the period I owned the car the judder was present on almost every drive. At first my wife thought it was my driving (yeah right; why has this never happened on any of the cars we owned previously? That'll be because none of them was a BMW 320i). When this started to happen when she drove the car she finally agreed it wasn't my driving!
I figured it would only get worse and could foresee more dissatisfaction for the BMW dealership network until the point where my car was out of warranty. By now I hated driving the car so it had to go while it was still saleable at two years old and 11,000 miles. It's still for sale at the dealership. I bloody well pampered that car and it's an absolute minter but I hope no-one on this forum goes anywhere near it.