…so, assuming that 30% off code proves too irresistible (in the face of overwhelming reasons to suggest I just shouldn’t) can I simply take the MicroSD card (with full paid-for OS maps on) out of my Garmin 800 and put it straight into an 810 or are they in any way locked to the device?

Yes you can just put the card in. That’s what I did when I upgrade. No problems.

Did you buy the maps on a MicroSD card, or on a DVD or a download?
If you bought the maps on the MicroSD card, then they are locked to that card. So they will work if you put the card in any Garmin device, but won’t work if you copy them onto another card.
If you bought the maps as a DVD or download, then they are locked to the serial number of your Edge 800. So won’t work in any other device.

I thought there was something like this that made an exception but couldn’t remember what it was. I got the maps ready loaded onto a MicroSD card that was bundled with a Garmin 800 so I am assuming that makes them portable then?