Hi, We have been lent an 18 month old roger armstrong bassinet from a friend. 2 other babies have slept in it for a total of about 4-6 months. Should I be getting a new mattress for it? Or is the one it came with still safe after other babies have slept in it? Thanks

Red Nose recommends that a mattress for a bassinet be firm, clean, well-fitting & flat (not elevated) & be no thicker than 75mm.
If you are happy that the mattress remains clean (where/how has it been stored) & doesn’t contain mould/stains or tears & that is remains firm & fits appropriately, then it should be ok to use.
https://rednose.org.au/article/is-it-safe-to-use-a-second-hand-mattress

You may also be interested in Red Nose information about bassinets.
https://rednose.org.au/article/bassinets