Approach them about it now while its still 'fresh'. If you leave it, they will start acrruing time towards adverse posession of your land. We had a bit of a ruckus with our neighbours last year as they had removed a hedge and replaced it with a fence that effectively nicked about 20-30cm of our garden the whole way down.
It happened before we lived here when my poor Gran lived here, she was in her 90s with dementia and there was no way she would have even realized, never mind confronted them. Some people are just cheeky and/or stupid and you should acquaint yourself with boundary law and your rights immediately.
In our case, I got a copy of the plot from the land registry, to show the boundary and shape of our garden. Fortunately, there were end wall boundaries to use as reference points and the plot width in our title deeds to show that the fence was too far on our side.
I know it sounds petty, but it does matter. I stewed on it for a year before I finally presented them with a little package of documents, including photos of just how distorted our garden boundary was from the end and even Google maps! They had helped themselves to a lot of our land and our garden isn't big enough to concede anything. The husband was a bit miffed but grudgingly accepted we were right. The wife, who is a vindictive piece of work, argued that my Gran was fine with it - that standard piece of boundary law!! Anyway, she was eventually told to be quiet and they moved it within a month - on what would have been my dear Gran's 102nd birthday had she not died a month earlier. When it was done, I almost cried, feeling like I'd defended my Gran from her horrible invading neighbours or something! Afterwards, it was obvious the fence needed moving and, even though it caused me a fair bit of stress, and the wife stomped around slamming doors and playing loud music and bitching for months afterwards, I am glad we raised the issue and got it sorted. I haven't lost anything, I didn't like them or their stupid barking dogs in the first place!
Don't let it go, they should have agreed a shared or party wall with you beforehand, otherwise it stays on THEIR SIDE. If you feel you can raise the issue verbally with them without a fight then fine, but I found myself swiftly attacked by my shifty neighbours, who are clearly practiced at talking their way out of trouble, and I was glad I had everything on paper and clearly written out to back me up. The phrase you need in a legal sense is 'occupying without permission'. Good luck!