Good luck, I hope they call you back and you are able to get it sorted.
If they refuse to be of any help, I'd recommend approaching Citizen's Advice or Trading Standards to find out where you stand, if you haven't already.
Also, I know you said you don't want to have to look for a new venue, but why not contact a few, explain the situation you're in and see what happens? With 5 months to go, you may be able to pick up a good last minute deal if a place has had a cancellation!
Plus you would get the satisfaction of telling them where to stick it, and don't run the risk of any negative feelings on your wedding day. I'm not saying definitely go for it - but it can't hurt to look!
You might even find a venue that is able to provide you with gf/df menus and you can compare costs, and report back to your venue.
Good luck again! X