Thanks everyone. I posted a reply to this, but it's stuck on my phone. How odd.
It was a new cupboard in December. I've tried bicarb, and squirting Purdy and Figg cleaning products on the inside, in the hope that they'd permeate the wood, but it hasn't made a difference.
I'm reluctant to paint it, as it would change the look of it, and as it is it matches other items in the room. I'll have a go at the vinegar thing, as I haven't tried that, and will try the soap trick too. I have lots of essential oils, so could try that - maybe if I mix some with carrier oil and rub it around the inside it might work. I could put a Neutradol thing in afterwards in the hope that it stops the smells from coming back.
Thanks again, everyone. Some good ideas here.