the sleepers still run, indeed the franchise was recently re-let and the new incumbents (Serco) take over from FirstGroup on the 31st; but, that too is expensive, and they don't operate on saturdays.
best way to travel to scotland without the 8hour drive and retain the convenience of a car (almost essential in the highlands), is to fly and then hire - for random dates in may, a mid-size car booked today via easyjet can be had for around £160