Thanks for the replies, appreciated.
So have gone and measured everything up and came up with something interesting.
So to start with, the scope is the Skywatcher 16 inch Dobsonian, the goto collapsible version.
https://www.ozscopes.com.au/skywatch...telescope.html
As you mentioned, it works out to be f 4.5, it is listed as a f 4.4
So when measured it is actually a f 3.5 scope.
The mirror is 400mm diameter, ( 16 inches )
From the mirror surface to the centre of the eye piece it is 1400mm ( 55 inches ) This is with the scope fully extended.
So it is a very interesting outcome, unless i have done the maths wrong, I have double and triple checked the two measurements, but fairly sure it all works out to be f 3.5