Thanks for all the helpful comments. From Hugh's information I can see my paper map is the OS 1:25k Dark Peak map edition A4, revised 2002, revised for selected change 2010 and copyright 2011. Curiously there doesn't seem to be a way to look on the OS web site and see the version/revision details for their maps.
Both SatMap and the Garmin Birds Eye are 1:25k, in the area where I know there's a difference the SatMap version matches the paper version. I visit the area regularly and OS/SatMap matches ground.
This is the problem area that I know of with, depending on how you count them, up to 5 differences, but to be fair there may be no more, from Birdseye version:
and this is the same area from the SatMap/paper version They are definitely both 1:25k, I've cut these from the screen and the slight difference in size is how they display on my screen.
At the moment I'm tempted to stick with Garmin given that the most recent SatMap really was a feeble attempt at an upgrade but then I'm rewarding poor customer service and poor instruction manuals whereas SatMap staff are amazingly helpful, they keep their maps up to date and their manual is like the encyclopaedia Britannica!
The SatMap people are coming to Cotswold's shop in Nottingham this month so I might go and see if they reveal any plans about software upgrades which will make the decision a little easier!