All you have todo to make 50p or £2 is to go to Tesco.com and signup to the Greener living club free of charge and they will credit your club card number with 50 clubcard points meaning you get back 50p, however you can convert these to 4x their value meaning £2.
Just thought i would share this
Sign up for Emails - Tesco.com
ps.
offer ends 6th april 2008