The plastic mounts for the limit switches I printed will actually slide down the rails and the extrusion if the gantry goes up against them (provided i use plastic slot nuts that I've designed and printed that will secure the mounts in place up until a certain amount of force). I could modify the print so that the rail carriages come into contact with the mounts after the limit switch is activated so it would slide the mounts and activated limit switch out of the way without damaging them. Behind this I could put a buffer as you suggested. I think this would work and if things did go tits up I'd notice it happening if I was looking. The mounts are cheap to reprint if they do get damaged. Next question is... is it worth using one off those PILZ relays to cut PSU/mains voltage off and does this stop movement quicker?
Thanks for all the advice.