Hedge between properties - vent and advice!

[minimods]

There is a hedge between my garden and my neighbour's garden. It was obviously planted on the neighbours side many years ago and has over time grown over the border onto my property. I tend to keep 'my side' tidy by trimming it with shears and keeping it pretty level. Today I have just come home to find the neighbour has trimmed the whole hedge, inducing 'my side' of the hedge. Now this wouldn't normally be an issue apart from:
1. They didn't say anything and obviously waited until I had gone out before accessing my property
2. They've left a load of hedge trimmings on my grass I have just had to tidy up
3. They've trampled on some freshly seeded young grass in the process, possibly killing some of it

Now am I right in saying that even though the hedge is planted on their side of the boundary, if it goes over to my side it is my responsibility?

The same neighbour removed a large bush bordering our properties a few months back, accessed my garden and left me a load of mess in the process, so it isn't the first time this has happened.

How can I broach this, I don't want a neighbours from hell type situation. Due to the distance between our front doors and aforementioned hedge I don't speak to the neighbour other than a friendly hello. I don't want cause any friction.