Outlet pipe overhanging neighbour's property

[Summerdays]

Our house was built in the 1900s. We bought our Victorian end of terrace house in 1996 - we added central heating in 1998. The house next door was built in 1950s and their land extends right to our boundary wall. We have right of access [it's in the deeds] to use the side path to access our back garden. This is next door's main entrance as they moved their front door to the side. *They are now saying that the overflow pipe from our first floor bathroom cold water tank is illegal as the overflow pipe protrudes from our boundary wall into the air space of the passage. It's about 30' above ground level. They say this is their property and their air space. *We pointed out that the overflow pipe was there when we moved in - back in 1996. They then said the other outlet pipe [this is our expansion tank added when we installed central heating] and even higher up as it's in the loft is also illegal.
I'm sure the cold water tank [and the outlet pipe] was in place when the neighbours' built their house in the 1950s. It was definitely there when we moved in back in 1996. I'm not sure what remedy they want as it's an end of terrace house so there's no way we can relocate the cold water tank and expansion tank. They are talking about invading their air space. They obviously have nothing else better to do with their time but this is obviously a concern to us. Any advice would be very welcome. *They are not reasonable people [ a bit odd!] so it's not worth trying to talk to them. I feel sorry for them in a way but would like to know where we stand legally. Many thanks