Wax jacket smell

Not sure where to put this, but I have a Barbour wax jacket that’s only about four years old. Now granted I didn’t wear it at all last year and that’s probably where my problem lies. I reckon I left it back in while it was still wet and so the smell started to muster. I had it under the stairs and only took it out recently and it has the most rancid mouldy smell, although with no visible mould mind you.

I adore the jacket and it cost a lot so I’m not inclined to get rid of it. Does anyone know how I might get the smell out of it? The instructions sewn into the inside of the jacket strictly advise against washing the jacket as it will remove the wax coating, and it can’t be reapplied. I’ve tried washing it down with a sponge and soapy water, and one tip online suggested hanging it in a bin bag with cat litter at the bottom to absorb the smell. Did both and it’s still ponging! Any advice?