[Arran]

I was thinking of some of the solvents they sell at Home Depot, like ketone, xylene, acetone, or cleaners like ammonia, I don't know what sort of stripper you used but most of them have at least one of those in it, other then ammonia. Ammonia used to be used to darken oak at one time. I think replacing the walnut veneer is the best option, whatever stained it up has gone through it, and it looks mechanically damaged anyhow. One more idea, have you tried steaming out the stains, a wet rag and an old clothes iron might be worth a shot?
Regards
Arran