Court should be the last resort.
Write to them and ask them to correct the installation within a reasonable amount of time say, 3 weeks, and you will pay on satisfactory completion. If they ignore you again write, giving two days for service, a letter headed “Letter before legal action” and ask them again to complete the installation in accordance with the contract within say, 28 days, referring to your previous letter/phone calls or you will be forced to take legal action.
Be reasonable English/Welsh courts like reasonable behaviour, it is very easy to make a claim and much help is at hand, in your claim show you have telephoned repeatedly and have written twice, you have taken every opportunity to avoid Court but this appears the only remedy left to you.