When you go to contract, Tiggs, your solicitor will have a schedule of questions to ask of the sellers.
One of these will concern extensions and work done.
You've found evidence of Planning Permission, BUT, more important is evidence of Building Regs compliance. Your solicitor will ask the Council about this, if you haven't found out for yourself.
You mention a previous bung-up that was cleared from next door.
I assume then, that your drain discharges into next door. Nothing sinister with that. Quite normal. What might be problematic is if it's the other way around, and next door discharges onto your property.
To start off, you just need some drain-tracing. Any builder could soon work out which way everything flows.
Rules have changed since 1985. Today, any drain serving more than one property (known as a sewer), is maintained by the Water Company, not you. Today, building over or within 3 metres of a sewer is possible, but must have the permission of the Water Company.
It is most likely that this is your drain alone. It simply used to be in the garden... now it's in the kitchen.
Yes, drains can be diverted (if necessary.)
So, for now, does the extension have Building Regs?
Where do all the drains run (very simple drain-map)?
Nothing problematic about internal manholes. Screw-down sealed covers. That's all. Not ideal, but not onerous.