They don't charge if they can't cool your air con down by 10%. How on earth does this work? 10% of 22º would take the temperature down to -7.5º!!
The only thing it could be is that they beat the ambient/internal difference by 10%, i.e. if it is 30º outside and the car can cool the inside to 16º then they'll give you back money if they can't get it to 14.6º
EDIT >
http://www.kwik-fit.com/about-air-conditioning.asp
"Let Kwik Fit recharge your Air Con while-u-wait for only £44.95. We will replace and recharge lubricant and refrigerant levels in line with your manufacturer’s recommendation. If we cannot improve the coolest vent temperature from your car by more than 10% - then you pay nothing at all."
So yeah - everyone get down to kwik-fit for free air-con service. 10% of the vent temperature on an absolute scale is LOADS. You simply can't take a percentage of a centigrade scale! Could definitely have them on false advertising. Stupid llamas.