you have accepted your leyandii caused damage
not planted by your good selves by any chance ?
I think this is covered by the law of delapidation
q1 Can I be compelled to share the building of a new wall as opposed to a fence, which would not cause a problem, as we will be liable for damages with a wall? - yes - from what you say - it is gonna be more than half
q2 Can we prevented from removing the trees on grounds of loss of privacy and loss of value?
unclear question if you re read it - but the answer is no
possibly
Can we be prevented from removing the trees on grounds of loss of privacy and loss of value? - to which property, your own ? no - theirs ? are you joking ? it is causing damage and if they prevent removal of the leylandii then they are taking on future losses themselves
possibly
Can we prevent the removing the trees on grounds of loss of privacy and loss of value? - since the trees you agree are causing collapse then I say no - esp the loss of value bit as the trees are in fact causing loss of value
In short you know what is wrong and what has caused it
and the simplest cheapest thing is to uproot the trees and replace/repair the wall.
in terms of the title: Forced into a position of civil liability for future damages ... - excuse me - did anyone hold a shot gun over you as the trees were planted ? No one has forced anything
read this and your blood will run cold ...
http://www.inbrief.co.uk/neighbour-disputes/damage-caused-by-roots/