PLW is absolutely right about building regs (as one might hope, he is a sparky after all!
). I must admit though, if it was my garden, I would rather install a proper armoured cable myself than use an extension lead for a long period.
I'd be less worried about installing a simple spur (which requires an hour or two of concentration) than I would be about leaving an unprotected cable outdoors for months at a time. I just wouldn't trust myself to look out for it for that long!
I'd be interested to hear what you think about the new regs PLW - are they a good thing from a safety point of view, or just a bit overzealous?