In reply to Graeme G:
You might do better to go and ask on Singletrack, those guys are always hauling their bikes up Scottish mountains.
My own experience of this was on Mount Keen where we used mountain bikes for the long walk in from Glen Tanar in the north east. They were perfect for that and allowed us to get well past the footbridge and about another mile up towards the ridge to the point where the bulldozed track stopped and there was a turning circle. My buddy stashed his bike there and walked the last few hundred feet but I carried and pushed mine as far as the footpath that bypasses the summit then gave up and walked. Even riding back down from there to the Landy track was a waste of effort with the rough terrain, so my advice would be to ride as far as there's a good track then stash the bikes and walk.