Unless things have changed, do a google on the Batley, Dewsbury, Heckmondwike and Cleckheaton area's of West Yorkshire, looking for Mattress/ Bed Manufacturers.
It used to be the centre of the universe for bed maufacture and even if some have gone under, there will still be something up there.
You could try Leisure Furnishing of Nottingham.
I had a warranty claim refused by my dealer and I contacted Leisure Furnishing who were the original supplier of our mattress and they exchanged without any problems.
Excellent customer service. Worth a phone call.
My daughter wanted a sprung single mattress for her caravan 3ft wide but only 6ft long and had one supplied by the shop in Somercotes .nr Alfreton, this shop is called "Bonkers", I can supply more details if required. It only cost £80, but it depends on the type and quality.
Changed ny avatar but any beady eye will see I had not moved the toilet door yet. I was just a bit short of photos as so busy changing everything else. Really nust find an uptodate photo. I originally used the original cushions but was a bit of a jigsaw puzzle so had new cushions made at uphosterers at Pinxton, Notts..Not expensive but sorry to say would have liked to have keptd to the original material but not available anymore. never mind.
I ordered a sprung mattress off Amazon last week and it got delivered 2 days later,they have a good range so most preferences should be covered. I spent the previous week hitting the shops,not only were they pricey but most told me 4 weeks to deliver!!