Good News! We can now reserve "cheap as chips" tickets on East Coast Trains. I have just managed to get three First Class tickets (with complimentary food & drink) from Montrose to Kings Cross for £37.93 each, including postage.
I managed to get this price by travelling as a small group (minimum of three).
Click on the "Special Offers" tab and the "Big savings for a small group" drop down option in the box beneath.
You can get second class tickets in a small group of three for about £13.35 plus postage. Second Class single tickets for individuals are currently showing as £18.00 for this trip. Not bad, eh?