Well I have struggled with this when Ollie started on water tabs and steroids - searched the internet etc.
Vet mentioned baking powder (soda) dry and rub in leave then hoover, thinking it was floor related urine, I found mention the lemon neutralizes urine so started adding juice to sponge to clean. I never went the baking powder or lemon mixed as somewhere the chemistry knowledge has left me
It is one of those things that when it gets up your nose it is a horror to get rid of.
I would spray with lemon water mix and take advantage of the nice weather and dry out in the fresh air, I would also leave the inner out in the fresh air and maybe in the garage overnight for a couple of days, the embedded smell I think will be on the inner cushion more than the cover.
There is a lemon anti-bac spray in Asda I got ages ago by Astonish used it to spray around if the smell lingered.
John my Ollie is over 14 and now he wears a disposable pad to catch any incidents ( poor lad dripples while he walks to go outside for a few hours after his water tablet
- so this saves me the problem a bit plus at night time he also wears one.
Do hope this helps
Julie