Thanks again. Got it working and wanted to report back the final issue/solutions.Cables are very finicky with the switch. A lot will not work. Start with trying the ones that came with it to make sure it works with your Mac before trying others. I posted earlier in this thread a few that are known to work as have others.
Firstly for the Windows machine, the culprit bizarrely ended up being the Docking station (HP Ultra Slim). When a DisplayPort to USB-C cable was plugged in there it failed to work, and even when the DP cable was plugged directly into the laptop's DP port (whilst docked) it would fail. Undocking the laptop and using it's own DP port works fine with the new (cheaper!) cable I'll link below. I've done a bit of research on updating the laptop's DP firmware, but so far that has failed.
For the Mac, the culprit was the Apple-supplied TB4 cable. Swapping to the cables that came with the Cabledeconn switch worked immediately.
So far, so good, and the switching is immediate with no obvious lag. The blinding blue light is mildly annoying in a bedroom overnight, but it is easily covered.
So the products used are:
Windows laptop to switch, (DP to USB-C): Cable Matters 8K USB C to DisplayPort Cable 1.8 m(USBC to Display port Lead/Displayport to USB C Cable) 8K@60Hz, 4K@60Hz, 2K@144Hz in Black - Thunderb
Switch: CABLEDECONN USB-C Type-C 8K Updated Switch USB 3.1 C Gen 2 Cable for Video Bi-Direction 8K@30Hz 4K@120Hz Power Delivery 100w 10Gbps Data Transfer Spli
A side note... they do a much less expensive version of the switch that doesn't have Power Delivery. Haven't tried it of course, but assume it's the same as above less PD. If someone were happy to just plug the MacBook into its own power supply it would save the extra money.
I should also caveat that I'm not using the full 5k resolution. The Windows machine maxes out at 4k (3840x2160 I believe) and on the Mac I find 5k too much for my aging eyes!