The build

What type of door is it, wood or UPVC. If the first that’s easy enough and reclamation yards carry old doors so a quick call should find something . The latter in a stone building is a bit harder as you need to measure from outside and measure its narrowest points. You infill the gaps then with expanding foam before cutting some thin plastic to cover the foam. There are a few miss measure upvc companies in South Wales where you can pick up new doors more cheaply.