Time to switch garages, I think.
Given that perspective, you wouldn't lose anything by having a friendly chat with them that goes along the lines '...been a customer for years, and now I am unhappy...it may be that we've both been unlucky, but I'm not happy with the outcome where you did stuff that didn't work, and I pay the total price for it...'
Even if they say "sod off and die...", you aren't in any worse position than you are now and they may offer something or another, such as a reduced price to fix the issue (I'd think that was the least they could do, but that's another matter).
In some ways, I'd like to advise you to pursue whatever legal rights you have, etc, etc, and ensure that the bad garages get weeded out, but, pragmatically, the costs of going down that route are likely to make it less than the best course of action for you.
Under the sale of goods act, goods and services have to be of 'reasonable' quality; against a minimum expectation of 125k miles life (and you get the feeling that a typical life would be twice that, or more, just Ford have set the limit for safety, rather than have a few very angry customers wandering around) something must be very wrong for the outcome to be 20 k miles life. They'd probably have a defence if they could argue that you mistreated it (how?), or that they could argue that you knew that you got an inferior one for a cheaper price and that it was made clear to you, at the time of decision that this was the case. Otherwise, I can't see that they have a leg to stand on, but even so, if they had a good lawyer and were determined to fight, they could make it difficult to prove to the satisfaction of a court.
Technically, someone further up the chain of supply (the manufacturer, the distributor) could be liable to them, and one of the things that they might suggest is that you prosecute, eg, the manufacturer. This would be hopeless as you don't have a legal contract with them, but if they do try to push you down this route, it could be hard to resist without deploying your own lawyer or solicitor.
There might be some mileage in just getting a solicitor/lawyer to write to them, but, if they call your bluff and say 'Well, let's go to court then' you are back in the same, not very good, position.