You're absolutely right, the problem is proving it.
I had exactly the same with a Mondeo (company car). Took it to Quick-Fit who rounded off the nut adapter and then came back to me going "it's broke, that, sorry mate." I know categorically that it's their fault, because a) they're the only people ever to have removed wheels other than me, and b) the adapter was perfectly fine when I changed a different wheel on it two days previously.
I rang the lease company, they basically said that whilst it's obvious where the fault lay, it's more trouble than it's worth to them to persue Quick-Fit and try and prove liability. They (leasing co) stumped up for the repair without quibble.
So, yeah. I'd say it's absolutely not your problem and the Ford garage should be dealing with it, however you might struggle getting them to admit that. What might be in your favour is it's all four; you could argue from the state of the tyres that it's highly unlikely you've removed all four wheels since they last had it. You never know, you might get a 'gesture of goodwill' out of them.
Are you looking for a warranty claim, or are you looking for them to fix something they broke?
Good luck.