I recently bought an end of terrace house and have gutted it, i have moved the bathroom from the front of the house into one of the bigger bedrooms at the back, when we connected up the new soil pipe and disconnected the old one we noticed waste coming from our old pipe, it appears that the next house next door (who only just bought it as well, they were both owned by same landlord) has their toilet pipe coming through the wall into our old bathroom and connecting to our pipe, this room is now going to be a bedroom and i really want to rip all this piping out as its old smelly and quite high up in the floor.
However this will leave next door with no working toilet, and a rather large bill to either re-locate his bathroom or somehow route the soil pipe through his house to the back (its mid-terrace), obviously this is not going to go down to well with him.
You need to establish whether there is/was any agreement for your property to dispose of the waste from next door - this may be in the deeds or may be a separate agreement. I think if this arrangement has been in place for more than 13 years it is established and you are going to have to take legal advice.
If you both have purchased from the same buyer, i would have a quick word with your solicitor's as this might be covered under the terms of sale. The seller might cough up rather than getting involved with the courts.
is the soil pipe plastic? if so and internal i can only assume your house isnt to old? if so then it is strange for two houses share the same internal stack (if it was external then common) was your house and next door once 1 house?
next door could have an external stack on the front, unsightly but an option. ( if building control would allow)
nxt door could consider a macerator, which allows you to put a toilet just about anywhere and pumps your waste away. these range from £150- £400+
i wouldnt like the idea of next doors waste discharging into my internal stack, is your new stack external now then? did you need planing permission (i think you do) so ask your building control inspector what he thinks