I don't understand how the fee comes to £450. The invoice above shows two site visits, one for call out, one to fit the correct RCBO?
The customer, should of been aware of the fees charged by this company, opposed the fees you would of charged. As been asked several times OP, why didn't your customer contact you to rectify the fault?
Faced with a bill of £450, it is not unreasonable not to expect your customer to recover the costs from you, as the invoice 'suggesting' bad workmanship, and in any case you installed the now faulty product. The Consumers Rights Act 2015 allows a period of 6 years to take a claim to the small claims court for faulty goods in England. Doesn't mean the goods to last 6 years. Some manufactures (MK) offer 10 year warranties on their products, not sure what Wylex do?
It seems this company has questionable business practises, but the fees are there to see, so it shouldn't have come as a shock to your customer. Personally, I would obtain some legal advise, either with CAB or an initial consultation with a solicitor, to understand your position.
Edit 'suggest to suggesting'.