Thanks both for the replies!
Just had a look at the last weekend in March and the fares are ^18.50 outward and ^16.50 back. The last weekend in April is ^13 each way.
So it looks like the ^13 fares still exist but instead of being able to book them up to a few days in advance, they would now have to be booked almost 3 months ahead?
I originally thought that this could be a result of a large decrease of allocation (meaning the very few available are snapped up quickly), but the more journeys I checked the more it became apparent that the ^13 fares aren't available on ANY journeys less than 2 months in advance. The chances of this being a result of the all the advance fares having been bought already is so slim as to rule it out.
This would seem to suggest that there has been a change in allocation policy rather than a just a reduced allocation. Whatever the reason, an increase of over 30% is really hard to take and would sadly make it impossible for me to make this journey so often.
What are the chances of getting
FGW▸ to explain what's going on here?