@Tommyf have you discussed the trespass with your neighbour or even advised him you have a problem with it?
You have three choices:
1 put up and get on with it, dealing with future problems and the loss of value down the line
2 negotiate with him in good faith and without prejudice to avoid legal action.
3 engage a specialist solicitor to secure an injunction and compensation.
Nothing stops you from starting with 2, but be clear about what you want him to do an acceptable remedy.
If you go with 3, expect costs of between £3 & 5k which you may or may not get back.
The council are doing everything they can to point you in the right direction.
If you are uncomfortable dealing with conflict - start with a friendly but firm letter along the lines:
Dear Neighbour,
Unfortunately the extensions to your property and modifications to my property without permission would allow me to bring an action against you for trespass. Having sought advice I am assured, I could gain an injunction forcing you to repair the damage and remedy the trespass in addition to any compensation and necessary legal fees. This will not end well for either of us, so I'd be very keen to discuss a possible remedy which minimises the impact to both of us.
etc.