Hi godthe
Doing the same myself,, Im assuming you're talking about a sewer drain...
went into the local council offices and asked. They showed me the main public sewer line outside on the road and said that as there was no main sewer line running through my back garden then it wasn't their concern. BUT be very careful if you are sharing a sewer line at the back as it can be a messy situation if anything goes wrong (legally too).
In our case the sewer line at the back of our house is shared between 4 houses that lead back out on to the road to the main sewer. The council consider it a private line so if anything goes wrong it's up to everyone in the line to sort it out.
As ours is at the start of the line then we can do what we like without causing disruption to any neighbours. Builders reckon it'd be straghtforward enough to move it.
A friend of mine had some sewer problems and had to get everyone to agree to chip in to get dynorod out to suss it out ..tricky situation..