Okay, let's assume that they know then. It's time to look at this realistically.
- There was an accident. They are now peeved off because their holiday was ruined (blah, blah, cry, cry) and they want someone else to pay their bill. They are scamming you - pure and simple.
- You were sent on a work trip and told not to do anything dangerous. You went skiing and the accident occurred on the beginner slope. You need to ask yourself, would your employer consider skiing on the beginner slope a dangerous act, or some kind of unreasonable risk? The answer really should be "no" here. If your employer thinks otherwise, then what about the colleagues that were with you?
- You have not admitted to any liability for medical or anything else. Keep it that way. The fact that they are not even pursuing medical is a mystery.
Basically -
do have any contact with them under any circumstances. Don't even look at their Facebook page - if they have any kind of smarts, they will know. You now have a much bigger problem to deal with, and that's the advice from your union.
Your employer has nothing to do with any alleged claim, but you were on a work related trip. These people are clearly using that and yes, what they are doing does constitute blackmail.
Your biggest problem is therefore the advice that you were given by the union rep - that being not to say anything to your employer. If these people do know where you work, then what do you think is going to happen when your employer hears about this from the scammers instead of you?
You need to sit down with your union rep again and you need to do it quick. Staying quiet is a massive risk and if these people do contact your employer - your screwed.