GOOD NEWS! BA have now got me back on my original flight!

Turns out it was a system error back in October when the routing changed and it didn't rebook me on the new flight and didn't alert.

Then when I paid for my seat reservation (now refunded) it revalidated everything and updated my booking to show the return as cancelled.

Although a complete mess up by BA their customer service has been very good and they have kept me updated all week by email or phone, although I wouldn't expect anything less in this situation.