Depressed at the cost of payg micro sims. I enquired at my local '3' store about occasionally inserting my iPhone sim into the iPad. I was told, '3' would eventually cotton on to this and disconnect my sim.
But then the girl assistant showed me how to sync the iPad with my iPhone, to enable me to use 3g.
Being of more...er...senior years, i can't remember how she did it. But am looking forward to trying it out.