There are such things called CCTV survey on drainage , People who share drainage normally have this carried out at a shared cost and then determin who is causing problem ! , I called a company called 1st drainclear in chislehurst they carried out a CCTV survey for ?120.00 + vat and to clear the blockage they only charged us ?60.00 + vat which was split between 5 houses now when there is a blockage all 5 houses pay a share. Hope this helps there phone number is 0208 467 7007