JackKnife, re what the doubles are that you see on video.
I think Mark Sullivans are......
600 Nitro by Marcel Thys
577 Nitro by Charles Osborne
500 Nitro... Sorry don't know the maker of this one
450 No2 Nitro by Joseph Lang
450-400 3 1/4" Nitro by Army Navy
Also there's the 700 Nitro by Watson Brothers which was offered for sale recently but at this stage I have not seen this rifle being used on video .
All these are of English make apart from the Marcel Thys which is from Belgium.
There are a few other makes of doubles used by clients on the videos including 470's by Rigby, William Evans and Westley Richards.