Going back to the "Room Supplement" debate that I and a few others had last month, its clear now that the FAQs and advice on the CS website have very definitely been completely changed/ updated on this - to exclude the possibility of upgrading on seated tickets with a supplement! CS clearly read this forum
The pricing of the sleeper continues to be a bit odd though - I've got some more work related business in Dundee next week and am looking to then be in London the next day for more meetings - so have been on the site today (hence discovering the updated FAQs!). It seems barmy that I can get the Lowlander from Edinburgh for £75 cheaper than the Highlander from Dundee for a classic cabin (£175 vs £250). The evening advance ticket Dundee to Edinburgh is just over a tenner! I'm guessing some of it may be "more cabins available" but a £75 difference seems ridiculous.
Have booked this provisionally as you can cancel up until day of travel according to Trainline business - I'm almost certain to get the Lowlander ive booked, but good to have the option for a refund if my London meetings don't happen for any reason.