I've never had any problems north of Inverness, but obviously some people have. Having a chat with the guard one day he said with a smile "Bicycles, yes we have our problems now and again"
I can't say I've ever had a problem getting the bike on the sleeper from London to Inverness either, but sometimes going south had to wait a day or two for a slot. The cycle compartment being regularly full of fresh fish, capacity is limited southbound.
Scotrail obviously get a lot of revenue from the all year round fish trade so are not so worried about inconveniencing some summer cyclists.