Take photos to show that its new work before mess is cleared up.
Get google earth screenshots if you can to show its new work.
Possibly rightmove will have old flat particulars online still for their flat showing before images.
Think if you can stay calm talking to them because as an owner you're stuck with your neighbours and need to be able to get along.
They have just spent out a chunk of money (even if it was done stupidly) and are likely to be on the deffensive if you go round to see them. This can come across as agression and could escalate if your understandably strong feelings are not contained.
I'd go round with a note. Keep it brief. I've arrived home and i'm shocked, my land etc. Dont want to go in heavy handed and report as you are neighbours and will incur heafty fines for them. Please rectify asap and confirm within 24hrs how you are going to rectify.