I have purchase two of these in the past and did the following
Trasferred and on number to both of them and got another £5 credit per sim. As I only use one of them the most I used their Ping A Pound a service to transfer one pound over from the other sim each day.
Basically I got £50.00 airtime credit for £20.00 delivered. Just bought some more so i can get some more 1/2 price credit
*** Also if you do not like the option of tarriff on that sim, just change it one of the other options.
Sorry meant to say transferred a number on to each one to get an extra £5.00 credit. Just quote PING1 when arranging the transfer.
Also if you phone is locked to O2 the Tesco Sim will work, because it is the O2 network. I have tested this in an O2 locked phone.