Simon and Cindy, so sorry to hear about the issues, but you can still go to YS Falls and I urge you to do so, it is beautiful. Monday is the only day they are closed and there are many other tour companies that come highly recommended.
I can put you in touch with Chicken, my good friend.
Check out his website here: www.chickensmagicbus.com
Or if you click on the Transportation link above there are also several other companies that are recommended.

Keep in mind that emails are usually a lot slower in Jamaica, meaning that you might be used to getting a response in less than 24 hours, whereas in Jamaica, sometimes it can be 3 or more days before receiving a response. A phone call is usually the quickest way to get a hold of someone in Jamaica.....or you could even contact them when you get to Jamaica. Don't give up, there are still plenty of options for you to get to YS....and even if you don't, you will surely have fun in Negril.

All the best! Have a blast! And we are all here to help if you need any other suggestions or recommendations! Just ask!