If I was in your position, I'd ask a dress alteration shop how much they'd charge to shorten 274cm curtains to 258cm ones. If the price is reasonable, I'd then head back to Dunelm to look for some cheapish 274cm curtains, as that appears to be a standard size.
An alternative source of good quality long curtains, which could then be shortened, is, of course charity shops - but I shouldn't need to tell
you that! (Most of the curtains in my house came from charity shops).
This Dunelm link filters results by 'longer drops', so that you can search for 274cm ones more easily:
https://www.dunelm.com/category/home-and-furniture/curtains-and-blinds/curtains/all-ready-made-curtains?f_attributes.productDescriptiveAttributes.ExtendedSizes=Longer+Drops
Needle & Thread clothing alterations are at 11 North Methven Street. Their phone number is 01738 248232 or you could try an email to
[email protected]
(The comments on their Facebook page include references to curtain alterations)
An alternative shop is Clothing Alterations, at 79 South Methven St. The phone number is a mobile one, 07394 428595.