The fact is, it's stupid that you can book a Bargain Berth from Inverness to Euston but not to Crewe.
However, personally, I would be pretty uncomfortable trying to leg it out of the train hoping no-one saw me!
If you ask me, ScotRail need to revise their Sleeper fare structure. For one thing it's mad that the fare from Preston to Inverness is the same as from London. First Class Sleeper Single includes Underground travel and First Class Lounge and shower usage at Euston, none of which you can use at Preston.
Also, solo travellers like me are not well catered for. If you don't wish to share, the cheapest way of going return to Fort William is £236 - by buying two Advance Singles each way and having the cabin to yourself. Or you can do it for £238.30 with an Off Peak Return and a solo supplement each way, but that's often not available each way. And before anyone says solo supplements are supposed to be one way only, the ScotRail web site will let you book one in each direction in the same transaction.
I know - you could buy two bargain berths and have the cabin to yourself but they're usually sold out.
And - you could buy a Standard Single for £100, and if you end up having to share then pay the £36 upgrade. However First Class may then be sold out!
I gather it's easier on the Night Riviera, but then, I don't wish to travel to Penzance ...