how does the saying go, with friends like that. it seems to me that you have been more than reasonable, you say you have no paperwork,hopefully you have a receipt for the £400 you gave him, if he is a motor trader you could report him to trading standards, alternatively write to him, saying that unless you receive a full refund within 7 days, you will issue a sumons at the county court. this is easy it costs around 10 percent of the amount claimed, but if the claim is found in your favour he has to pay your costs. good luck it sounds daunting but it realy is simple dont let him get away with it