Vorn all the way. not cheap but they really are the business.
Comes in 3 sizes, fox, lynx and a bigger one (can't remember the name off hand). I have used a lynx for about 15 months and wouldn't be without it. Rirfle is carried centrally and totally protected , yet can be released and in the firing position almost as quickly as un-slinging.
The video on the Vorn web site show the user grabbing and retrieving their rifle over their shoulder but as an oldie i'm not flexible enough to do that. However its just as easy to reach round to the small of your back and grab the rirfle by the stock.
Gregor, I see you are in the Borders, i'm near Peebles if you want to have a look at the packback