4 years. Not much good to me when they cannot fix or claiming to fix it only for problems to reoccur.
At this stage, I'd be insisting that they take the car back.
There is a questionmark over why you've held the car for so long, and that will be used against you, in your discussions - with the likely result being that you will probably have to agree some form of compromise, if exchanging the car for another.
You need to start with a very clear conversation with the dealer (who sold you the car) - tell them that:
* you've lost complete faith in the car, due to continued problems,
* you don't consider it to have been of merchantable quality when sold, but appreciate that's no reflection on the dealer, just an unfortunite car.
* you've given them a number of opportunities to repair it, but it's still not right, so now you want a replacement or refund.
* Be factual in the letter, summarise the faults, the timeliness, periods you've been without your car as a result etc.
They won't be happy with the above, but if it's a fair reflection of the situation, then you need to say it very clearly, before proceeding.
If you find it difficult to have that full conversation with them, and ideally with a witness present, then also put it in writing, but go to them and try to discuss it, only if that isn't too your satisfaction, then hand them the letter.
I'd give them 7 days to respond, in writing, if you've to go the letter route.
In the meantime, you need to consider what your really want here, and what you'll settle for. A full refund, or brand new car, might be a bit of a stretch, given this has gone on two years, but puts the place to start, I suppose.
I'd also be preparing a letter to the main distributor, to include copy of the letter sent to the dealer, calling for them to intervene.
Mention of the Joe Duffy radio show is probably left for a heat of the moment conversation, I don't think I'd put it in a letter though, even if you do intended to phone the show.