Hi Westv
It might be this problem I had in 2012 (text of key post copied below). Basically, the seat re-selection screen just "hangs" if you've picked the last seat in a particular ticket price band:-
http://www.railforums.co.uk/showthread.php?t=62498&highlight=littletorelate&page=3
"OK - here's what's happening on EC's website ('cause I just tested it to prove it).
You're lucky(?) enough to grab the last ticket available in a particular price band. (A punter wouldn't normally know this of course - I checked the number of tickets on offer by retrying and "upping" the number of adult tickets requested by one each time.)
You click on "Buy Now" and go through to the seat reservation screen. You try and change your seat allocation and get an error message saying you can't (as someone has noted above for the last ticket of a batch). I've tried several times and it always does this.
So you think "oh, there must be something up with my browser" (well that's what I thought). So you close it down, open it up again, and, hey presto, that ticket price has now disappeared from the list of those on offer.
Thanks for all the pointers on what was happening - it was v. useful. I'm still going to complain to East Coast. This behaviour and result is unacceptable in my opinion. I'll let you know..."