Wheels damaged at manned car wash.

I'm pretty particular about washing my car so usually do it myself but sometime when I don't have the time I use a manned car was at a local garage. I've used these guys about 5 times in the past year with no issues until today.
After getting home I noticed that the paint on the wheels was damaged, it's pretty obvious that whatever chemical was used was either not appropriate or mixed too strong.
I went back over to complain to the lads there, I showed them the damage and said that the wheels where not in that condition before the car was washed. They said that they could not have caused the damage. I asked them to show me what was used to clean the wheels and the guy pointed out an old unlabelled 20L jerry can which they water down and use in a pressure sprayer.

Just wondering where I stand, what's my chances if I take this further? Has anybody any experience chasing something like this up?

I'm going to put a letter of complaint together and send it on but doubt it will have much impact.

I'm fuming over this, I should have gave the car a good check over before and after the wash.
You can see the damage in these pictures.










Here's a picture of the car taken on the 29th of June, it's clear that the wheels were not damaged then.