If you own a Peter Guild sofa mattress and you need the mattress replacement for wear and tear, Life and Health Section of The Guardian has this tip:

Peter Guild no longer makes sofa beds, and replacement mattresses are hard to source – the assumption seems to be that people just buy a whole new sofa bed.Martin Cave at Peter Cook International, a supplier of furniture components and accessories, is optimistic that he can help, provided the folding mechanism (ie, the number of times the mattress folds in on itself when you put it away) matches what they have in stock.

Via Peter Cook International