If it's shared, then the drain may have been adopted by the Water Company. Not in every case, but still worth asking.
Water Companies will have a sewer plan they can refer to. Well worth ringing them.
Otherwise, I would wait until it happens again. It needs a few measurements and a bit of detective work to find exactly where the problem is, rather than simply clearing the blockage until the next time. Have an experienced local builder look at it rather than a surveyor in this case.