Help with choosing hedging plants please

That's correct

That's, also, technically correct but it does become illegal and has to be cut if the local authority say so. Even if it's on private property. If they serve the order on you (they, first, normally just have a chat and ask you to cut it) and you don't cut it then they can come along and cut it themselves and serve you with the high cost for them doing it. As with any contravention of most Acts, failure to follow an order can result, eventually, in hefty fines.

It would be the council that make the decision as to whether it constitutes a nuisance under the Act and that is simply the decision of the enforcement officer (you don't really get much say in it although they ask you your side of the problem). Some councils have very strict enforcement and others are more lenient. Of course, it wouldn't be for me to say that some councils actually publicise the fact that they do this adjudication simply so they can get a further income stream from their charges.

In England some councils impose the law without exception (but very few) and some homeowners have got around it by having their 'hedge' inside their property and not on the boundary. This can work OK if your garden is big enough to do so. In a town near us I have been asked by the authority, as a non-interested party, to give an opinion on the situation in some cases (they're not trying to pass the buck but wish to try to feel as though they are being as fair as possible).

I've no idea of the situation in Scotland