Hello, my name is Jim and I am a qualified lawyer happy to help you today.
The Consumer Rights Act 2015 applies here which states that you can expect the service to be carried out with reasonable care and skill - and that the product (the clutch) needs to be fit for purpose and free from defects. If not then this is breach of contract on the garage's part.
The garage is completely wrong to charge you for another clutch. The lack of warranty has no effect on your consumer rights either.
I would send the garage a letter before action to demand payment within 14 days and say that if they do not pay you, you will issue county court proceedings against them. See attached template as an example of what to say.
Assuming they refuse to pay, you would then need to issue a claim using the simple procedure in Scotland here:
For claims over £300 (up to £5,000) the court fee is £104. If you are in receipt of certain benefits you are exempt from paying the above fee. The fee exemption can be applied for at the above website.
Once you start the claim the Sheriff will register the claim and a timetable will be given to you to detail how the claim will proceed. For this size of claim you can easily do it yourself without a lawyer (or come back to this site for further guidance).
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.