Three micro-sim in new iPad but no 3G signal - how do I activate?

I bought a new iPad from John Lewis last weekend and recieved a free Three 3G sim card with 1 month/1GB worth of free data. I have inserted the sim card in to the iPad and the network and signal bars are showing in the top left hand corner but I have no 3G connection. My mum got one at the same time and she also has no 3G.

I have activated the iPad, synced it with itunes (having insured all software/operating system updates have been done), turned the wifi off, made sure all of the cellular data settings are switched on but still have no 3G. I tried to register the sim card but this involves logging on to the Three website via 3G which I cannot do.

Can anyone offer any suggestions?