Also, I went back to the property at the weekend and couldn't see any
evidence of the damp on the walls (survey done in August) and the
survey didn't say where the water was coming from, so I'm beginning to
wonder how bad it is.

Damp is only a problem if it actually manifests itself in something bad,
like a musty smell or wallpaper/paint self stripping. If you have neither,
then save your cash.

Tanking is basically just sticking bitumen on the walls. If there is room,
you should probably follow the bitumen by celotex insulation and
plasterboard, which should help reduce condensation and save energy.

And yes, you should fix any blocked drains.

Christian.