Bunch of bar stewards. It’s straight up criminal damage. Police won’t be interested, but you are entitled to civil remedy. But:
although i can’t prove this
Here is you problem. Ask around, chainsaws aren’t quiet and someone may have seen or heard something. You could also try calling around local tree surgeons – are they of a size that your neighbour would really be taking the tops off and disposing of the trees themselves?
With regrads to “proof” any court would probabky determine beyond reasonable doubt that it was them that cut them down.
Based on no evidence at all other than “they live next door, m’lud”? Not going to happen. The bar for a criminal offence (which you refer to) is, in reality, very high and this will fall well short. That it is completely obvious is not a legal argument.
In civil court, where the test is “on the balance of probabilities” then you may stand a chance. You could really do with a witness though…
NB you yourself may be in breach of high hedge legislation and end up being required to reduce the height, but there’s a proper process for that via the council.