Whilst I don’t disagree to take legal advice, you paying a solicitor is unreasonable as it’s not your issue.
you are best asking citizen advice or if you have legal help on your insurance go through that
on the face of it though, unless your mum had a party wall agreement to this affect she signed, or it is already in the deeds thst there is “easement “ across the land registered in you/your mums name for the purpose of maintenance , then the neighbour has no legal grounds to demand it
they can ask you to include it on deeds as a new easement right - I’d refuse point blank. It could be a real problem for sale and house value to some people. They can ask the new purchasers to sign an easement (usually they would need to pay a sum in compensation) but the neighbour could refuse and there’s nowt the person can do. It is your land and no one can insist on easement rights being given if they don’t exist already, or there’s another legal document stating access
check your and their deeds first to see if you can see any easement rights or covenants or party wall stuff that maybe indicate they already have rights to your land. It’ll cost you around £6 and take 24-48 hours if you go on line. You just need postcodes and numbers of each house. Dead simple.
id also be inclined to go into plans submitted to planning department and see what plans show as gap ? I’d be a bit surprised if building right on boundary isn’t clearly shown on plan. Check the plans vs boundaries. Make sure he’s only built on what he got planning consent for. Again you can get online form council with a bit of digging.
if I can’t see anything in either deed or on planning consent suggesting they have legal access already, I’d then , at next asking, simply respond to neighbour with what legal document they have to claim access for maintenance already? If they look bemuse or say there isn’t one and that’s what they’re wanting, just shrug and say sorry you are not going to do that. End of matter.
The fact they’re “wanting new buyer to “sign a right of access” (easement) strongly suggests he has no legal rights . In which case tough shit. Should have thought about that before you built.
you certainly should not and cannot agree to your perspective buyer signing anything to give easement rights. Thst could land you in a complex legal situation around misrepresenting if neighbour tries to use it with new owners to say you’d given verbal contract or informal precedent etc.
And in meantime do NOT allow them to access your land for maintenance. Absolute not. Close up any access gap with boundary fence . Immediately .
once you’ve got the info re deeds and planning consent, as I say get advice form citizen advice or your insurance legal aid, to just check your rights.
I had a similar thing with a previous house. New buyer came in to property next door (that also owned a business next door to thst) and wanted me to “sign a few legal things about his deeds - just admin and he’d cover costs and deal with all solicitor” in return for him chopping back a nuisance tree I’d asked him about . And he wanted me not to object to his planing consent for extension. I checked his deeds. Turned out he wanted to revoke the covenant he was burdened with by our land holding not to operate a business on his land. And probably had intention of expanding business onto the property next door, We hadn’t actually realised we had benefit of this covenant. Stupid twat for thinking I was a little stupid womin who wouldn’t understand. He kept pestering and saying “if you want me to cut tree back then this is how it works and it is give and take”. Told him if he was that passionate about tree I would of course back down and just cut branches on my side. He was not a happy bunny when the fellers arrived on my land 🤷🏼♀️🤣🤣