I’d like to be able to swap easily between an old Garmin 305 and a newer 800.
Is there any sort of adapter available so I can just use the one mount for both devices ?

Yes, sram do one, think they are about 3 quid or so,wiggle did sell them and may still do, i had one and it worked fine

http://www.wiggle.co.uk/sram-quickview-computer-mount-adaptor/
out of stock but i think they are on ebay amazon etc

Thanks Swampi, that’s just what I’m looking for.
My only doubt is that they describe it as fitting any “SRAM quarter-turn Quickview Mount”.
It looks the same as a Garmin 1/4 turn mount…

…so why don’t they say it is compatible ?

I’ll get one for £5.15 from ebay if someone can confirm it’s definitely the right thing.

Yes they fit fine,i used them, the wings have a little indent in the middle and that matches the garmins to locate the unit centrally,even the bryton fits the sram mount the the proud bit of plastic the locates in that indent damges the brytons a bit as they don’t have the indent and the 2 raised locating pegs are slightly diff, just go for it

OK thanks, I’ve ordered it.
The old slide and click mount never seemed as secure as the newer type to me, so I’ll probably glue the adapter to the 305.

Yep, I got one a few weeks ago as Garmin have apparently stopped doing the older mounts, and I was down to one between 3 bikes. Works very well.