If you use the ability to change allowance every month there is a potential for big savings. These are some examples of the of the average extra cost per month for different allowances using this (this image on the Kitz forums shows how it is worked out):
The one that really stands out is the 300GB where they are asking an extra £10 per month but by flipping to the 2TB one month in four it actually costs an average of £3.75. If they increased the allowance on the £10 extra to say 750GB it would make a lot more sense (although savings could still be made by flipping)!
AAISP November 2016
(Previous ISP Plusnet November 2001 to October 2016) Why I left Plusnet
Telephone rental: Pulse8