Hiya, is it just the vinyl, the foam, the base pan or all three that need replacement? nearly £400 for a seat is a bit heavy!
I got my recovered locally very reasonably and he modified the foam to suit me at the same time. Unless your foam is knackered throughout or the pan is rusted out, I'd do that. Or even get a genuine cover and let your local upholstery guy fit it professionally for you. I'd even look at a second-hand seat and get that recovered for about half the cost of that replacement seat.