OK - I've compiled the most recent version of 4.8.y from GitHub - raspberrypi/linux: Kernel source tree for Raspberry Pi Foundation-provided kernel builds. Issues unrelated to the linux kernel should be posted on the community forum at https://www.raspberrypi.org/forum (4.8.16).
Can't seem to reproduce the problem and I now have a more up-to-date kernel :-).
The problem is when I try and find the 1st version of 4.9.y with the problem I can't get any earlier than 4.9.10 as it doesn't have the necessary rpi specific patches (arch/arm/configs/bcm2709_defconfig) to compile.
I'm trying a compile starting at 28cad4892910 (the earliest point in the rpi-4.9.y branch with this file) to see what happens, but it doesn't seem that much earlier than the 4.9.10-v7+ build I first tried which I already know has the issue.
I did wonder about applying rpi patches to base 4.9 (69973b830859bc6529a) or maybe an earlier version, but that's beyond my git skills at present :-(.
More than happy to add another remote to my git repo and give it a go if someone can publish a link...