as above-go and get it back,its either something to do with the new bloke or not.
if what you state above is factual you have no contract verbal implied or otherwise with the new gaffer-you will need to persevere -go down with a trailer/transporter and a witness or three and copies of documentation showing you own it-and a spare key if you have it-if they dont comply phone the police and report it stolen and tell them you know where it is and you are trying to get it back
i wouldnt pay a penny for the work if car has lain for ages,you have lost the use of it etc,not your fault he has been locked up