DO THIS AT YOUR OWN RISK
iam not sure is this the same for everyone, but there should be no risk if you dont mind the hassle (you have 14 days to cancel if things dont work out)So here's what you do.
1. Take the existing customer special deal 200 minutes + 12 GB. You need to actually sign up on this contract. http://www.three.co.uk/existing-customer-offers/sim-plans-for-phones/sim-size?campaign=exc&step=confirmplan&planid=1400628966677&?campaign=exc
2. Go to three change of plans on their website and you can select different offers. the main offers that i think are a very good are:
- 400 extra minutes for the price of £1
- 18gb extra data for the price of £3
or you can simply stick to 12gb + 200 minutes for £12
iam not sure will cashback or refer a friend will work, but you can give it a try for an extra discount.