There was a place in Walsall called The Tent Hire Warehouse that used to do repairs. Worth giving them a try. https://www.a2zcamping.co.uk/
The Canvas Repair Centre are situated in Chesterfield but operate a collection and delivery service which costs £30. Repair costs will, of course be on top of that. https://canvasrepaircentre.com/
I know this is not your part of the world, but I just had a really great major repair of our well used (and abused) mountain tent from CIT Camping in Walton. Really good service including sourcing new poles.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.