I often find that booking it in stages is cheaper. For Kings Cross to Leeds go to East Coast's own website. Always the cheapest. My in-laws are in Doncaster so I use them on each visit as its the same line. For home to Kings Cross I tend to book via National Rail Enquiries, Trainlie to Red Spotted Hanky. Hope that helps!
Travelling from shoreham. Been on trainline and and southern only, usually do car or coach but need to be in leeds by 12pm and having a beer or two, so the train it needs to be.Thanks
Don't forget to look at 'GroupSave3' or 'GroupSave4' tickets if travelling to London as part of the journey, and there are 3 or 4 of you who can travel off peak for the price of 2.