Vodafone tethering charges - how to I avoid?

[ICW1]

Vodafone have currenlty given me a 3 month download as much data as you want trial.
It ends on 18th Sept.
I am going to France next week they told me for £3 a day i can still download as much as I like.
BUT..... I cannot tether or it will cost me an arm and a leg.

Question1. How do they know if I tether (i.e. is it because I set up a personal Hot Spot?)
Question2. Is there a way to get around this so they don't know

thanks

 

[chunky]

Jailbreak and use the tether me tweak.