Although I still didn't find the shrike, I did get good views of a sparrowhawk, a cormorant heading towards the reservoir(s), large flocks of long-tailed tits, a few smaller groups of small finches (I'm guessing goldfinch and linnet), and a small group of twitchers / photographers down the hill towards the canal. Since they were all settled but chatting , my guesses were a) that was where the bird had been sighted most often and b) it hadn't showed yet.
Right on both counts. But with my 45 minutes having already stretched beyond the hour (boy, was I going to be in trouble) I spotted it on a wire above the canal, not far from where this group had set up. A quick jog of a few hundred yards and there I was, locked on to a great grey shrike just 20 yards of so away. I enjoyed it for 10 minutes or so, and crept away for family duties and a where-the-hell-have-you-been telling off.
Worth it though ;-)
Bird of the day: Great grey shrike (Lanius excubitor), a stunning predator that is a real autumn / winter gem for UK birders - rare, but not so rare that one can't turn up on pretty much anyone's patch given a bit of luck.