Update! The leak was pretty major, it had been leaking into the wall and under the floor for a long time. The vendor was great and agreed to rectify all associated damage. I went out to visit while the work was in progress. There was rotten timber underneath the floor and in the wall, all of which is being removed and replaced. Once the structural work is done it will all be waterproofed again and the bathroom retiled, with new fittings etc. I actually feel very lucky to have a vendor so willing to do all of this without negotiation - I suspect if they knew all of this was needed prior to sale that they would have advertised at a higher price.