Scotsgirl,
So far, our consultant has had to have a 'weigh and go' day once in 6 months as she had training for her job and couldn't rearrange it. It should very occasionally happen if they can't get someone else to cover it, but not all the time!
Personally, if my weight loss was slowing down, leaving group would be the last thing I would do! It's at times like these when you need all the motivation (and help) you can get to keep losing and not giving up and just letting the weight pile back on...
Your consultant will have loads of resources to try and stop this 'slow down' - you just need to ask!
Good luck...