The S&K seems to get good reports from people who have them. It doesn't require drilling the action, because it mounts with a pin in place of the rear sight and wedges into the charger bridge. It doesn't mount the scope as high as some, which is one of the challenges of the No.4 action. (I have one that I haven't tested. It's on a sporter No.4Mk1* I recently acquired. I'll trade it to you for your rear sight, if you like.)
The ATI is similar but seems to get more reports from users not happy with it holding zero.
I have a couple of Cad-Techniks and they work as advertised. I'm not sure if they are still available and I got both of mine from the U.S. long ago before current regulations made almost everything a potential problem to export from there. They also do not require drilling and they allow you to keep the rear sight in place. They do aggravate the problem of the scope ending up fairly high, but I am content with the Beartooth Comb-raising kit on one of my rifles and a replica No.4T cheekpiece on another.
If I didn't have the Cad-Techniks already I'd try the Addley that cantom suggests. It offers what I like in the Cad-Techniks but seems like it may do it better.
