ok - then you are not liable for the cost.
They fixed it and it only lasted 2.5 months
If they fitted inappropriate parts, they need to do it as their cost.
If not, then you can seek a Court order to compel them.
You would need to complete form N1:
The Court will send them a copy and see if they admit or deny the claim.
If they deny it the Court will set the matter down and decide whether to order them to correct the work,
Under the Consumer Rights Act 2015 they must act with all reasonable skill and care. Parts fitted must be of suitable quality.
Clearly, they are not.
Can I clarify anything for you about this today, please?
If not, I would appreciate a 5-star rating for my answer. If you need anything further I am available for a follow up at no extra cost.