You now have two options:
1) Ask them to repair the car but don't turn up to collect the car on the day that they are expecting you to, instead after a couple of days go down at 3AM.
With a spare key, drive away your newly repaired car & let THEM try suing YOU!
Or
2) Get a written report of the current work needed, from three separate garages/ mechanics. If it turns out to be a problem directly related to the work carried out then yes, go to court. Bring your invoices from 6 months ago with you, along with your engineers reports.
Be aware though, magistrates aren't mechanics, it might be worth getting a friendly mechanic to write a statement outlining that "they believe, in their professional opinion, that the problem is directly related to X or Y part"
X or Y part will be on your invoice.
Do not however go to court without evidence. You will look like an idiot.
Also be aware that it will take atleast six weeks for your claim to be heard, you will need to think about repairs in the meantime anyway.