Hi June

I note that you state that the mattress would have to be bent slightly, but you might find that if the mattress is taken up so that it lies diagonally across the stairs with the bottom edge against the bannister side and the top edge against the wall, that is will go up without bending.

If however, even at this angle it is tight, then you may wish to consider the purchase of a zip and link set, which comprises 2 x 2ft 6 inch by 6ft 6 inch mattresses are are sold in cases where access is an issue.

Having said that, if the gap diagonally is not too tight, a naturally upholstered product may go up as these are more flexible and can bend slightly without damaging any internal components. So i would be looking at the Artisan range.

Before buying a mattress, I would suggest you call our office and we can then give more detailed advice based upon any additional information you can provide. I hope that this helps.

Kind Regards Gary