To be totally honest, why would you want a replacement cover not from a manufacturer?
Does this not technically "modify" the product and make all liability of using that equipment the NHS's problem.
At least if you use the manufacturers' parts and accessories, and you have a failure.. it lies at there feet if the items are at fault.
I know this will open a big can of worms, but if your trust/hospital own pressure Area care mattresses, surely you have a maintenance budget? Cost should never be an issue when repairing a product.. as doesn't that ultimately effect patient care?