I have a secondhand Silver Cross pram and need to buy a new mattress. The Silver Cross ones from Babybirds are £25 plus postage - I can see why they are more expensive as they have the stitched crease where the sit-up bit in the pram goes. Is this necessary, or does a £11 foam crib mattress from Mothercare work just as well? Anyone who has used one, please let me know! Thank you