So any Yak herders who are looking for a C5 Weather-cheater here is the report for the size 'M'.
Today I tried on the Weather-cheater so as to keep the herders happy.
I have girth of just under 100cm and the cheater fits very loosely, I would guesstimate that a girth of up to 120cm would be no problem.
For those herders who have a girth in access of 120cm there are three choices.
1. Look for a cheater in size 'L'
2. Lose weight, or
3. Forget it.