It has just occurred to me that I forgot to mention two very important things in the last post.
The first is that I tried twitching the bait, two maggots on a size 16, half way through the period when the perch started to feed. What was happening was this; I noticed that the very tiny crucian like nibbles were happening just after the float settled and that if the bite failed to become more positive then after a while the bait would be ignored, so I started casting across to the far bank and every thirty seconds giving the reel handle a turn to sink the float and draw it a foot closer. The bites not only became more frequent but also more positive.
The other thing that stood out about this particular session is that all but one of these ten perch were lip hooked fairly and squarely, I only had to operate on one small fish, using small forceps to turn the hook having found that disgorgers really don't work very well when a perch has gorged the bait. This episode of lip hooked perch stands in stark contrast to my usual experience of canal perch, most of which have tended in the past to take baits straight down the gullet with predictable consequences.
Maybe it was a one off and next time they'll revert to usual habits? Who knows?