Via Quidco, you get £4.50 cashback for activating a tesco free sim card.
http://www.quidco.com/tesco-mobile-free-sim/

I ordered the sim and the packaging mentioned dialing a number to register the sim for extra credit when you top up (the tv advertised triple credits). This is a free call which can be done straight away. Now here is the crux, this call activates the sim and triggers the cashbask. No spend required.

For Tescos benefit, this sim triple credit deal seems great and hopefully they'll get enough new customers not to close this loophole, but grab it quick whilst its there.

Regards to all

Bugeyed