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Thanks for the reassurance, folks, I actually knew a fair bit about the little blighters having Googled (but hadn't come across chijiki's suggested site - thanks!)
I guess what I was looking for was some indication as to the size of the problem, ie I was always told that fleas never come singly, if you find one then you blitz the place because there are invariably more, but wasn't sure if this was true for bedbugs. Comloulou's right, the bedbug (or wall louse, Cimex lectularius) is a very specific type of creepy crawly - microscopic dust mites are one thing, but bloodsucking brutes a quarter of an inch long are something else entirely! Definitely bigger and uglier than fleas.
I shall take everyone's advice and try not to panic too much - and await the inevitable headlice, which are doing the rounds at my daughter's nursery too!