Witholding Personal Property, is it legal?

I find myself in the middle of a potential problem between and old friend and a garage…
August last year he was driving his vehicle back from Toulouse to the UK, suffering a breakdown on the autoroute. Inside the vehicle he had an old Terrot motorcycle that had been stored for him for many years, along with his wife and adult children.
The vehicle was recovered and taken to tne recovery agent’s garage. The engine was beyond repair and the decision taken to scrap the vehicle. His roadside assistance provider also concurred with that decision.
His cover provided for a taxi to the airport and flights home for all.
I got involved because he asked if I would collect the motorcycle for him. I phoned the garage to say that I could not do so for 2 weeks, I was leaving that day to go to England. We did call in to collect a couple of cases of wine that they had to abandon in the vehicle. The garage owner got a leg of pork that could not go with them.
On my return from England I went back to the garage and had an excited garagiste screaming that I could not take anything until she had all the paperwork for destroying the vehicle.
Roll forward a few months, my friend has finally sent the V5c and the correct CERFA document. He had only just purchased the vehicle and had not received the V5. He spoke to DVLA about it being scrapped in France, so they just did the necessary on their computers to mark the vehicle as s rapped. It took a while to get the V5 reissued so that it can be scrapped in France.
So, to my question, I do not trust the garage that well, online reviews suggest that they look for every opportunity to squeeze more cash out of a scenario. I do not suggest, nor think that this has anything to do with nationality, some people are just like that. Anyways, I am concerned that when I go back with the paperwork they will try to hit me for storage charges, this having dragged on so long. Are they entitled to hold on to the motorcycle until any storage charges are paid, or should they start a civil claim against my friend in the UK?

Firstly let me say that I am not a lawyer, but it would seem reasonable for the garage to make at least some sort of storage charge as they seem to have had the vehicle taking up space on their premises for some time. I think your friend would do well to check with his roadside assistance provider as to whether they will cover a reasonable storage charge incurred as a direct result of the breakdown. When you do go back to the garage, it may be helpful to have a letter from the owner of the motorcycle giving you authority to take possession of it on his behalf. Hopefully a reasonable compromise can be found that gives each side at least something of what they want.