Original poster here... thanks for the comments... I'm still enjoying this car, and I still haven't had that error message again, so for now, I'll leave it.
I don't think I mentioned it in the review, but another niggle I had was with the alarm, the fob simply stopped working... a quick google suggested it needed reprogramming, but to be honest, after all the hassle I read some people have with these alarms, I actually just disconnected it instead, and now use the key to lock and unlock. The key appears to have a button, but this does not unlock the car unfortunately.
I also get some squeaking / friction from the steering wheel when turning it.
I've just been messing about this morning. I read about two free little mods - the golf tee mod to keep the exhaust flap open for a better noise at low revs, and a supposedly 60 second job of taking the muffler out of the airbox for a throatier sound. Well, I discovered that the golf tee mod had actually already been done! The 60 second job to remove the muffler turned into a 1 hour long job, taking out the entire airbox, which was straightforward enough, but getting it back on (connecting it to the MAF) was an absolute nightmare, but I've done it now. Truthfully, the difference isn't quite as pronounced as some say with the sound, but it's definitely given it a slightly harder edge at high revs, and maybe sounds a bit more throaty - not a huge difference, but it is just about noticeable. I'd love to hear what this engine would do with a good cone filter and exhaust combo, but to be honest, the car sounds so nice as it is, I'm reluctant to do much to it.
I've just been out on a 20 mile hoon, just for my fix. I can't get out as much as I'd like these days due to financial constraints, but suffice to say, I always love this car when I go out in it. Such fun to drive, and that engine just sings in a way no 4-pot ever could.
As for the petrol situation, I was under the impression that no fuel these days had high sulphur levels... mind you, I don't generally use supermarket fuels as a rule, but then again I did fill up at Sainsburys once. To be honest, its had a compression test recently, and that came back fine. Even if it only lasts 3 years, at £1800 it will have owed me nothing - superb car, and a lot of car for the money - it's a struggle these days to even find small superminis worth having at that sort of price, and I'd really much rather drive this than a rubbish Vauxhall Corsa or the like.
My next car may well be another Beemer, I've got a soft spot for the Z4 so I may go for that next... or perhaps an E46 M3 or E39 540i or M5... once I've got my money sorted.