Extension built on ajoining wall

[Juliedh]

I bought my house 16 years ago, with a one story extension on the side using neighbours extension wall which is on the boundary. The 2 extensions were built approx.25 and 26 years ago, with verbal consent between the owners at the time. There was no Party Wall Agreement
I now have new neighbours who moved in 6 months ago, and I have just replaced my existing roof that was leaking (no other changes). It was done while I was on holiday and I have come back to a letter from new neighbours saying they thought there was a fence between the 2 extensions?(Shouldn't t his have been picked up on survey) and they no longer give permission for their wall to be used for my conservatory/or roof. (Their roof edge/tiles of their extension do encroach on my land)
There has never been a problem in all of the 26 years that the extensions have been there, and all I have done is replace a roof and now it has become a problem. Any advice please

If nothing showed up in their survey, they would have a very hard time indeed overturning a decision made 26 years ago.
I can't see how you would need their permission at this point at all, they bought the house as seen.

You need to talk to a solicitor, but I think 20 years is enough to establish an easement so their permission is no longer required.