Garage has caused extensive damage to my vehicle

[Leese7]

Thank you for reading this, I am currently in dispute with a garage who is now claiming they are no longer trading and have admitted they are unqualified.
I own a classic, historic vehicle that was in need of some major repairs. The garage in question responded to my request for repairs via social media and we agreed the work would be undertaken. Initially things went well, the engine swap went smoothly and was paid for. The garage then began work on re-wiring and welding, however during this time the garage notified me that the windscreen had been broken whilst they had attempted to repair it. (We had discussed the windscreen as it was in need of re-sealing but the glass was sound. At no point did I give the garage the permission to attempt to fix the windscreen as I knew this was a job for a specialist. The garage is claiming I gave them permission to try) It is unclear if they even attempted to fix the windscreen or whether it fell out. The reason I believe this is the case is that there has appeared major damage to one side of my vehicle, including an original handle (the vehicle is 43 years old) the side trim, the rear brake light and damage to the nearside fibre-glass body work. The garage has admitted to the damage although he claims it happened when he was not at work. The most likely situation is that he collided the vehicle with a wall and this caused the windscreen to fall out as it was in a fragile state. The windscreen is now in smithereens and beyond repair.
The garage is now withholding my vehicle and has barricaded it behind several scrap cars, demanding £450 for the wiring and welding work before he releases it. He left my vehicle outside, without cover and filled the vehicle with his own scrap metal. I understand I can pay "with protest" and will do so if necessary to get my vehicle back but my dispute is that I am now left with approximately £1800 worth of damage to repair.
I wondered if anyone can help me with an official response to his demands as he is now threatening to remove my vehicle to a hidden location.
I have only received an "invoice" via text yesterday and it has no payment details, no name or address and no details of what I would potentially be paying for.

Any help constructing a legal response to this man would be really appreciated.
I fear going to court will not be fruitful as the man in question is now on benefits as he proports to be sick.

Thanks in advance.

[des8]

As the owner of two classics my priority would be in regaining possession of my vehicle.
Whilst he retains it and leaves it out in the open it will only deteriorate.

I would be paying the £450 demand at the point the vehicle is released.

Having recovered the vehicle I would then initiate a court claim.(there is no other legal response which has any teeth)
He may not at the moment have the wherewithall to pay, but he might eventually


Insured?

[Leese7]

Thank you for your response.
I had come to that conclusion too, so it is good to know I am approaching this in the right manner.
I do have insurance but sadly had reduced it to third party as bus has been off the road for some time.
I will pay "with protest" and go from there.
Many thanks