Why is he threatening to do this?
I am a solicitor in Glasgow.
He cant take the veranda back. It now belongs to you as it's attached to your house. If he refuses to do the snagging then you get another contractor to do it and deduct the cost from the amount of the final payment and you remit that to the builder only after the remedial work are done.
I'm afraid JustAnswer doesn't allow expert solicitors on its site to take on customers as clients but you can ask the Law Society to recommend someone in your area to help you. www.lawscot.org.uk for contact details.
Reasonable time is correct as a matter of law. However, reasonable in this instance could easily be argued as 14 days. Send them a letter saying that if the defects aren't fixed within 14 days, you will be terminating the contract, getting another contractor to do the outstanding work and deducting that from the sum sought by them as well as an amount for inconvenience and additional time spent.
I'm not allowed to recommend specific solicitors but in my experience the solicitors firms in Kilmarnock that do civil court work are more than competent for this kind of work.