Try (emphasis on the word "try") to get hold of your county council plan showing the location of all utilities in your area. We got ours eventually, from the Highways department.
Apart from gas and electric services, it should also indicate the line of the sewer/drain and whether (by colour coding) if it is deemed to be 'Public' or 'Private'.
I've had to go through a long drawn out process on a similar issue regarding the line of a surface drain which the council deny exists - despite a new public road servicing some new builds, all with drains!

If it's a public drain emptying on your land, I would have thought there would be some kind of easement which should be lodged with the deeds to your property.