Hi Holly,
Thank you for your enquiry.
The mattress required for the guest underbed is W75cm x L180cm x H17cm, the suitable mattress we sell is the compact single Europa comfort guest slimline mattress which can be found on the link below:
https://www.time4sleep.co.uk/products/europa-comfort-guest-slimline-mattress-compact-single
Kind regards
Laura
From the Time4Sleep team