Bill looking at the frame tech spec for this frame the angular geometry is the same for both the medium and the large frame.
So with particular respect to the seat tube so long as you have your seat height set correctly your hip to knee to pedal triangle will be the same on both a M & L.
The chain stays are the same length on both as well but the M wheel base is 978 mm and the L 995.
So if the same fork is used and the same head angle (the former I assume is and the latter is so as stated in the tech spec) then your large frame is 17 mm longer than a medium (about .7 inch).
Your seat pin set back looks to be about 15mm so switching it around moves the saddle centre 30 mm forwards.
However because you have an L I would have imagined your stack is higher that you would have set it on an M which would make up for the most of the 17 mm.
I'm not saying that you don't need to reverse your saddle pin but theoretically your tendency to move forward is more likely to be down to your handle bar each, i.e. the stem length and not the seat pin.
However if you have your front wheel raise this will move you saddle back from the bottom bracket centre line as measured on the horizontal.
Or perhaps you have a low thigh to calf length ratio.
I would have expected a Scot to have long thighs though so he can have deep pockets!
View attachment 335557
There you go again....
