Mattress

Can anybody help me please?
How do I get the smell of wees out of a mattress?
Unfortunately I didn't discover the wet bed until later in the afternoon yesterday so it had a good chance to soak in. I sponged it thoroughly with dishwashing liquid and water then dried it and sprinkled with baking soda, covered in a towel to try and absorb the odour.
I left it for a few hours then vacuumed up, but it still smells. DS has been sick so it is a strong sort of ammonia smell. (could be to do with the pregnant nose too, but it is quite noticeable and offensive!)
Normally I'd put it out in the sun to air but the weather is not cooperating.
Any other suggestions, please?

i was going to suggest baking soda... but since you've already tried that - orange and lemon pure essential oils are natural deoderisers as well, they should neutralize the wee smell. Mix a couple of drops with water in a spray bottle and spritz it lightly over the area then air as much as you can. don't use any heat to dry it, cause i've noticed with nappies this can "set" the smell.

good luck!