Turns out that visiting www.tesco.co.uk today in (undisguised) Opera gave me a message telling me about Tesco Access – a project to let people get to Tesco using alternative methods (Internet TV, phone, etc.). It wouldn’t let me any further without my registration!
Switching to fake IE mode let me straight into their site to do what I wanted to do.
It’s a shame Opera disguises itself by default – I’m sure nieve webmasters would think twice if they saw it in their stats or had a flurry of complaints from pissed off Opera users. Hard to call really. Still, useful Opera has this ability in the first place!