Having a potential legal right is one thing and enforcing it is another. I doubt Trading Standards would take up a case like this and is it worth the hassle of the Small Claims court where there is no guarantee of winning?
Sticking an honest but poor review online might be the best option but given their poor attitude to date I doubt the garage will respond or rectify the problem.
It’s hard to see where to go next and you may just have to put it behind you unfortunately. Just a thought, if you paid by credit card they might help you.

Lucas stopped making alternators in the UK back in the early 90's, no idea where production went.

Both Caterham's were fitted with Lucas alternators and they never lasted long, 3 years at the most, a few weeks if you were unlucky, always kept a spare. Some time in about 2004 I picked up a "new" Lucas unit, I needed more amps when I fuel injected the car but it did not last long but at the time I did not have a spare higher amp unit. Picked up a disgusting looking Valeo unit off an ancient Transit from a sc***py, what a revelation 14.4 volts charge after start up, the Lucas ones never managed any more than 13.7 volts. Unfortunately the local branch of the auto electrical factors I that supplied Lucas units closed and their nearest branch was about 70miles away, Lucas sat in the garage for a few years. Eventually decided should get the Lucas looked at (did not expect the Valeo to last forever) and wife took it to the local auto electrical reconditioner. Took one look at the Lucas unit and commented that they should be prosecuted for selling them as "reconditioned" or even "new". Took it apart and showed her how all the internals were well worn or simply knackered. Tested rectifier box which was kaput. Charged her £10 to sort it.

In 2010 a chap up our street gave me a Denso unit off a 1980's Daihatsu Charade GTTi. They were the alternator that everyone was fitting so despite it looking disgusting I made up some new brackets, added a wire for the warning light and fitted it. Its still on the car 13 years later, still gives 14.4 volts after start up (drops to 14.1 when hot running) and makes no noises.

Sold Valeo (which was still working fine but looked disgusting) and Lucas (never been fitted after local chap repaired it) on e-bay (about £30), bought spare Denso unit (not as disgusting as the one on the car) off e-bay (£14).

But even Denso suffer from companies selling r****** copies that fail and produce low volts. The ones on e-bay are mostly fakes, the "Nippon Denso" sticker gives the game away, Denso stopped using that name in the 80's or 90's when they started making more parts for European motors.

There are parts for some cars which have been unavailable for 2 years.

It's one of the reasons insurance has gone up so much - it's costing a fortune in hire cars whilst cars are off the road awaiting parts.

May I ask the name of your auto electrician GB.

I live close to Wellingborough