I just bought a new mattress on ebay it was a cancelled order , a king size mattress should have been �279 bought it for �109 with free delivery. Iordered it yesterday and is due to be delivered tomorrow. They had all sizes but only for next few days[ apparently] but still seems a good deal
I would get it from argos but I do not know the delivery time scale is on them. If you need it asap you have to really contact a local store or go in the store itself to see them some deliver same day!
If it is just for guests I slept on a really comfortable inflatable bed for a number of weeks recently it was a double height self inflating - plugged into mains outlet. The double height meant it was very comfy, natural height and did not deflate.