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Ok, Thank you.
I will review it and let you know my additional comments at some point today.
The good news is that the contract says that "Time is of the essence". The bad news is that it does not specify when the works needs to completed by. I note that you have communicated with the builder by phone (hopefully by texts) and you have enough evidence that they failed to complete it before the agreed date.
In your letter before claim state that the time was of the essence under the contract and they failed to complete it by [certain date] and demand the refund.
I have made my offer here.