East Coast know about it and have confirmed it to a number of other customers as a known issue. It's actually a pretty tricky problem. To secure availability of an advance fare on a particular train, the booking engine must make a seat reservation. When you try to change your seat, it then attempts to make a new seat reservation before releasing the old one. But if you had booked the last seat available at that advance price tier, it can't do this and you end up not being able to change the seat.
Thanks Indigo2. At the risk of boring everyone with my nerdy amateur systems-analyst hat on, I'm not sure you're right here?...
For a first class ticket, the seating allocation should surely be constrained only by the total number of available
physical seats in the first class coaches, not according to the no. of first class tickets available in a particular price band? I reckon it should work like this (for first class tickets in this example, although the same process applies to standard class):-
A customer selects a 1st class ticket type with reservable seating (of whatever price/flexibility type is available, and they wish to purchase). They then click "Buy Now". At this point the booking engine arbitrarily reserves an available 1st class seat (irrespective of the ticket type & price band).
The customer then goes through to the seat type/no. selection screens, where they're able to change the seat allocation if they wish (although possibly with limitations according to what's already allocated to other customers - e.g. there may be no more window seats). In the case of it being the
last of all the first class ticket types with reservable seating, then clearly the customer would get
no choice (wouldn't be able to change the automatically allocated seat at all).
I agree that in order to change a seat, the system has to first reserve the new one selected before releasing the old, but it can always do this provided there is of course at least one spare physical seat available in all the first class carriages.
In my case, there were plenty of vacant first class seats shown on the seating plan in the seat re-selection screen (plus a few occupied ones). This is as one would expect as this was the last of the block of
web-saver priced tickets only, not the last of
all reservable first class tickets on this train. Indeed, this screen
would allow me select a new vacant seat, but it wouldn't then
confirm the new selection when I clicked "Confirm". It just returned an error message (and I tried several times for different vacant seat positions).
It was this that led me to close my browser down and re-enter the system from scratch. And then of course I was locked out of getting the web-saver rate at this point because it was the last of the block, and still reserved in EC's system (although not then visible to me).
I reckon this seat-allocation bug is actually quite serious. There must be a fair no. of people who either accept a seat position they don't want, or who lock themselves out of that price-band by resetting their browser and buying the next band up thinking
another customer has snapped up the last of the lower. Consider that if there are (say) ten 1st web-saver fares allocated per train, and customers book two tickets each. Then this will happen for every fifth customer. Not an insignificant number! - and the cost difference can be quite large.