Fix the scratches and never, ever, ever let anything touch the surface of your car that you can't be 100% sure is perfectly clean and unlikely to scratch the surface (i.e. your own brush that you have kept from touching the floor or any dirt/mud)
There is probably a notice about using the facility at your own risk, but that doesn't absolve them of responsibility for damage/loss caused by the negligence of their employees.
You would need to show that Tesco or the operator of the machine (whoever gets the money) were negligent in either not removing foreign material from the brush on a regular basis, or not inspecting it for damaged bristles/vandalism (is there a schedule in place that someone isn't keeping to? Did some vandals push nails into the brush since the last check maybe?)
Bearing in mind that the obvious defence would be to suggest that the contaminant most likely came from the mud & sand covered 4x4 that was washed after a heavy off roading session immediately before you used it (or even your own vehicle if you didn't pre-wash it thoroughly to remove any grit) I'd have thought this would be difficult, although I suppose you could try a small claims action for reasonable rectification costs and hope that they won't defend it.
(You would need to contact them first, and send a Letter Before Action detailing your loss and why they are responsible for it)
