My 15 yr old Papillon has suddenly started weeing in house.  She isn't drinking excessively and appears well. She had some teeth out 3 weeks ago, so I wonder if it's anything to do with after effects of the anaesthetic. She had blood tests for liver and kidney function which were normal. Any ideas of what I can do?

She may have a urine infection.   Often the first sign of this, is weeing in the house ;-)

Personally, I'd want her seen by a vet.   In the meantime, make sure she's drinking enough.

She may have become incontinent due to her age.

I agree with Dill.  My papillon bitch started weeing in the house when she was quite elderly, having previously been very reliable indoors.  She had urine problem and stones which the vet removed.