I'll bite just once more then agree with Richard - I have to take issue with s&r over whether I'd get a police warning. I'll quote the original:
"Saturday night, I spent a couple of miles stuck behind a middle-lane hog, driving past an empty inside lane and unable to get out into the melee of the outer overtaking lane, despite flashing lights etc. etc. When the outer lane finally cleared, I was able to pull out and I stayed alongside the hog until we came to another, slower middle-lane hog (not totally sensible, but to give him an idea of the holding-up effect he was causing). What did he do? Pull into lane one and pass on the inside, only to pull back into the middle! I was so stunned I hadn't managed to complete the overtake and pull back in in time to balk him."
I flash my lights as a warning, the correct use if I recall (NOT to let people out of junctions or say hello!), then pull out to overtake, which I choose to do at a leisurely pace since we are approaching the speed limit. By the time we reach the slower car, I am alongside and slightly ahead, so not in any blind spot, then continue in the same vein while the other car completes the illegal undertaking manoeuvre. So who gets a tug from the law again?
Admittedly, the last line might have suggested an unacted-upon suggestion of further mischief, but not something I would do in the presence of an idiot like that.