Many thanks for your patience. You certainly do have an argument that you should not pay the full price, although the fact that there has been no response by them for 12+ months does not mean you can further reduce your potential liability. It is entirely u to them if they wanted to reply to you or discuss any offers on payment – as far as the law is concerned they have 6 years in which to legally pursue an outstanding debt so whether they take 1 week or 3 years to reject your offer and respond, that is up to them.
What may happen here is that you still have some joint liability for the outstanding fees. On one hand they did not advise the fitter of the correct specification so some liability would lie there. However, on the other hand they were instructed that the specification they were ready to install was the correct one. Even though that was given to them by someone who did not necessarily know what they were agreeing to, it would have been reasonable by them to assume that if that person was at the house at the time and did not in any way make it clear that they did not know what they were agreeing to, that their instructions would be valid. That would likely create some liability on your part and the final result could be that the liability is split (not necessarily equal proportions but not far off either).
As far as it stands, the outstanding amount is £600. You could still try and offer them an amount to settle, such as £200-300. If they are adamant that they want the £600 then you may as well take your chances with the small claims court. The worst case is that they get the full £600, plus interest which for a year would be around £50. You may also have to pay their court fees which are approximately £140. However, you would not have to pay their legal fees if they had to use solicitors to make the claim as each party pays their own legal fees. The best outcome would however still be a settlement before any legal action takes place and you should try and do this, but of course if you are also adamant that you do not wish to pay anything, then you may a well have your day in court as the risks are not that great in the small claims court.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating (3, 4 or 5 stars) as that is an important part of our process and recognises the time I have spent assisting you. If you need me to clarify anything before you go - please get back to me on here and I will assist further as best as I can. Thank you