How do I get rid of cigarette odour from leather?

[Velvetee]

I have a flat, which was let for a year to a couple I understood to be non smokers. However, it turned out they were both heavy smokers, so my flat had to be redecorated.

Anyway, the flat had a leather sofa in it and it's in reasonable condition, except that it smells like an old ashtray. I've had it cleaned, but the nicotine has obviously impregnated the skin completely.

Does anyone know how to get rid of this disgusting smell, without damaging the leather?

Thanks.