What's this piano worth then? This is certainly not my area of practice so I speak not as a lawyer but I agree with those who have suggested that you first port of call is to pay him a visit. No threats but I certainly would cut him no slack simply because he has disabilities.
If you can prove that you own the piano (original purchase receipt) then should you have no joy I would consider issuing County Court proceedings against him in the Small Claims Track (on the presumption that we are talking under £5k). You would not get your legal costs back even if successful so unless the piano was worth a lot I would not instruct a solicitor (not economic) but the Small Claims Track is designed so you can run your own claim without the need for solicitors. The CAB are normally quite capable of providing sufficient advice to help you through it.
If he isn't trading as a limited company then its harder to ascertain his financial status but iirc there is a site which will tell you if he's bankrupt if you do a search. If he is, forget it and chalk it down to experience.
A visit may help you ascertain if he is likely to be good for the money if you do sue him - there's no point getting a judgement against a man of straw, although you could stop him getting credit if you felt vindictive. Bear in mind though he may own his own house and you could get a charging order against it.