For instance we've been up Pen-y-ghent today and it reported 8.8 miles.... but if this is the horizontal distance then I guess we have done nearer 10.
This tickled my interest.
My hunch was, it would not have made such a big difference as that.
And, for a change, I was right.
Let's say for argument's sake you walked from Horton in Ribblesdale at 230m, up to Pen-y-ghent at 694m.
I plotted a circular route and MM reported a total ascent of 470m (and, predictably, a descent of 470m too), on a 9,080m ground track.
For the sake of simplicity (and to keep Mr Pythagoras happy), think of it as walking the hypotenuse of two triangles of base 4,540m and height 470m.
(... insert squaring and square-rooting here...)
So, the hypotenuse up to Pen-y-ghent would be 4,564m.
Giving you a total of 9,128m walking the slope. The slope only added about 400m to the ground track.
It seems that, if you just add the height difference to the ground track, you get a pretty good approximation.