We had a similar situation when we bought our second house. The previous owner left furniture and other belongings in the loft, shed and garage. There was a lot of it and we needed the space for our own storage. After 6 months he still hadn't removed it, always had an excuse 'didn't have a vehicle, nowhere to put it, got a bad back' etc.
We sent him a letter (didn't use a solicitor) giving him 30 days to remove it or it would be disposed of by a house clearance company and we'd send him the bill. I also sent him the name of the clearance company (after they had removed it) if he wanted to retrieve anything. We had no response from him and it was all removed as arranged, though I doubted he would reimburse us. One day DH came home to find an envelope on the mat stuffed with money, no message or anything, but it was the exact same amount as the bill I'd sent him so we knew who it was from. We never heard from him again. Out of curiosity I asked the clearance firm if he had collected any of his stuff from them, they said no. So I guess he just couldn't be bother with getting rid of it himself.