This feature has been around for months!
0/10 for observation then

. Now I know it's there I'll take more notice in future!
Yes its not that new and I find it very useful.
The invitation to upgrade to 1st can also ne useful, though not as useful as it could be..... if you see an EC service with no such recommendation then check out the extra manually as it may well be less than 5quid
I use the eastcoast booking site, but my regular journey/s don't involve East Coast trains, and don't offer 1st Class

.
I was being t-i-c about the panicking btw - it does look like a useful enhancement, and presumably updates instantly by collating all ticket sales across all sources?
Might be useful if I want to buy an Advance from a ticket office at a station that doesn't have a TOD machine, as if I check before leaving for the station and it shows there are plenty available for my chosen journey, it's unlikely they will all have disappeared in the 20 minutes it takes me to get to the station, so a stronger position for having the counter staff co-operate with my request. (In theory anyway

.)