I am having the exact same issue! I made the mistake of letting my son sleep in my bed whilst dh was working away. He peed the bed. MY bed! Cant replace mattress as it is a super king size bed and mattress is quite expensive! Have used Rug Doctor urine eliminator in the past and that has always worked so it might be worth you giving that a go, but nothing is shifting this odour at the moment! URGH!
when we had that rare spell of hot weather in the summer, i took ds2's mattress outside for two days and threw jugloads of water & Febreeze on it! luckily there were no stains on it anyway - but it smelt much better after I'd done that!