Hmm - I think I shall go out and get white wine vinegar - just in case it changes the taste. Another recipe had mentioned black onion seeds - are they readily available from the supermarket?
Last years chutney was green onion chutney and it used shallots - so thats why I got them again this year - though my toms are red and yellow this year. mmm - may go with red onion too and freeze shallots.
I got my black onion seeds from Sainsbo's - just a Schwartz jar, so imagine they are easy to come by. I didn't have plums, but added fresh fennel and fennel seeds, but I don't think this would go with plums. You could try allspice maybe.