Ive got a garmin 810 and was thinking of buying the 820 but note it doesn’t have an sd card slot. As I currently have genuine OS maps on a micro sd card for the 810, I’d like to use them on the 820. Anyone know if this is possible through the garmin centre or something?

OS maps that came on the card? If so I doubt you will be able to use them. On the other hand the OSM maps that come on the 820 are good though not perfect

Yes the os maps on the card.

Does the osm maps show details like bridleways, forest roads etc like os maps do?

I’ve just upgraded from an 800 with the OS maps to the 820. The in built maps seem fine to me, only done the one ride so far, followed a route round the North York moors no problem, showed all bridleways etc.