OK, got my sim last night from the local Asda, unlocked my old Nokia (using instructions via
moneysavingexpert.com article) after tea, registered my top-up card (they gave me 25p on it so I could make a call, which you have to do before you can receive calls apparently) and am up and running as an Asda payg customer. Can still pop my O2 sim back in if I change my mind of course.
Best signal strength I've ever had at home, so am happy with the experiment at this point. Looking forward to 8p/min calls and 4p texts - had to phone the plumber on my old sim today and the credit dropped like a stone.