From experiance through work, administrators usualy favour private individuals when trying to sort out the mess, and in most cases will 'do the right thing', all be it after a short delay.
The car does not belong to the administrators, it still belongs to the owner, he needs to prove title, but the administrators will be interested in any money owed for works carried out, and will be reluctant to 'help' until the bill is settled.
Reporting whatever it is stolen does not work, the ploice are not interested as they say that you handed the item over to them in the first place. Because of the above paragraph, Kidnapping would be more appropriate.
A car sales group went bust around here last year and shut 3 main dealers overnight, lots of local tradesmen had vans etc in for service and had to make do without them for several days whilst private customers cars were released quicker or transfered to other dealers for work tio be completed.