Paid large car repair bill but not fixed

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I've had a nightmare with my car, I took it to the local garage (who came highly recommended) for some diagnostic work. It was losing power at times when pulling away, very sluggish and didn't feel safe and also had problems starting the car at times. It has now been in the garage for weeks as they had lots of problems. They eventually told me they had worked out what was wrong (needed 4 things doing) and cost was roughly £800. It now looks like it had only fixed one of the original faults and it is still sometimes losing power.

I am at the end of my tether with it now. I've been without the car weeks, it's cost me a ridiculous amount of money (i don't have any savings or spare money each month so this is a huge repair bill for me). The garage say they have fixed it but one fault seems to still be there. I really can't afford any more money, I'd struggle to even pay for diagnostics elsewhere right now. I don't really know what I can do!

We had to have our exhaust fixed to pass the MOT. Catalytic converter, so very expensive. 3 days later I can hear the exhaust blowing... took it back and got it fixed - no additional cost.

So my questions are:
Did they do what they said they were going to do?
Did they charge for the things they said they were going to do?/did you get an estimate of costs?

If they did what they said they were going to do and charged what they estimated it would cost - then you could take it back if what they said they would do would fix it - it may be that it needs tightening up/gasket replacing/something else that should have been done at the time.

If they have done everything and checked it was done properly then something else is the problem and you're going to have to have it looked at again and pay for it.

If they didn't do what they said they would do but charged for it - take it back.

If they didn't do what they said they would do but didn't charge for it - you're going to have to get it fixed, there or elsewhere, and pay for it