I just hope, after the current neighbours, that we don't end up with another self-absorbed, entitled prick who thinks he can do whatever the hell he likes.
I think it's pretty obvious that that's exactly what you are getting.
Ok, speak to your solicitors about the issue overall but here is my advice. DON'T BE NICE.
I have lost count of how many threads I have read where nice folk like you have said ok to access, use of garden for scaffolding, all sorts of stuff that they didn't have to agree to at all but they've done it to be nice in the hope that the neighbour/builder/whoever would appreciate the gesture and it would improve relations. BIG MISTAKE - it almost ALWAYS ends up with said neighbour or builder totally taking the piss, because they think they can walk all over you.
I would honestly say that no matter what your solicitor advises, have as your start point a big fat NO FAVOURS. This bloke sounds like an entitled arsehole already and I would put money on it that he's going to cause you all sorts of grief taking the mick wherever he can get away with it.
So start from the point that no, you are not going to agree to a bloody thing which will put you out (hey, you might even get him to move on and create his havoc elsewhere!)
No clauses in lease - why the fuck should you agree to anything that isn't a. for your benefit and b. paid for handsomely?
NO to access. They will royally take the piss if any permission is given to use your property and no, they won't make good the damage. Endless threads on this. Access is their problem and no way are they coming into your house. If he wants to discuss that any further - and you suggest he doesn't even bother until planning permission has gone through - then he'll be paying through the nose for it.
Fuck no to any planning agreement. You can bet that what you will see in those plans will need adjustment if not outright refusal. You'll be considering them and any objections as you see fit.
The final point - yes you are not his bloody inferior, which is what he seems to think. So do yourselves a favour now and let him know in no uncertain terms that he won't be calling the shots when it comes to your property or anything which affects it, by doing all of the above!