Isn't it worth leaving a spare key with a friend somewhere in Cambridge anyway? I agree you don't want to have to go to them every time you put the bin out and the door swings shut, but it's less faff than a locksmith if you do manage to leave your key somewhere, and in an emergency you can always ring them up and say "I swore I'd never ask you to fetch something for me, but I'm trapped in a kryptonite mineshaft. Could you use the spare key and bring me the batterang on my sideboard?"

I admit, when leaving a key with Tim became an endless faff, but mainly because we both like pedantry, it wasn't an inherently bad idea.

For example: Laura, I have been admitted to A&E, please come and visit. Laura takes spare key, goes into my house, arrives at hospital with spare pants and my embroidery! V good. Forgot bunny slippers though.

Yes, exactly :)

(I didn't say that because you might end up giving your key to your closest (in the euclidean metric), rather than someone who'd be sure to visit you at the hospital. Besides, I liked the batcave idea, though admittedly, not as much as I liked the bunny slippers :))