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It can be hit or miss as you say. Maybe for the return leg it is an option as you'll be so knackered you could sleep anywhere.
If travelling as a pair the berths are a lot better value compared to solo as you can no longer get paired up with a stranger in a sharing one.
Cheapest I've seen is £200 for 2x each way, so 100 each. Not that much more than a seat at £80 odd (assuming that was 1 way)
Sleeper seated isn't too terrible. It is a reclining seat like an airplane. Better than a normal train seat by far.
Usual direction is S. to Glasgow.