After 4 weeks, they came in and said the treatment had been successful (despite me finding 1 or 2 of the buggers every few days). The diagnosis was based solely on my bed - the bed was clear, and since they will always be close to where the host is, that meant they'd been dealt with. By 6 weeks, I was still finding them everywhere but my bedroom - though it was perhaps 3 or 4 a week. Then it became a couple a week - and always in the bathroom, just above the laundry bin. Turned out that the last batch were living in a bag of unwashed clothes in there, and were getting out on the days where the cat was knocking the lid off slightly. Emptied the bag into the washing machine, and a bug dropped onto the kitchen floor as I did so. Squashed it, washed & dried the load at high heat and since then (almost two weeks ago) I've never seen another one (touch wood).
Bed is still clear.
Interestingly, my neighbour had recently found a single dead bed-bug in his bed and he saw the council pest control van in the street last week. He stopped one of the guys loading stuff back in the van to ask if they'd been at another flat in our stair. Turns out they were dealing with multiple flats in the block adjoining ours - apparently there was a massive infestation in there in all the flats. Now, my bedroom is on the adjoining wall, as is my downstairs neighbour's bedroom.....
Might explain where mine came from. Wonder if I could sue the housing association responsible for the properties to get my £350 treatment costs back?