Hi all - just to report back
Contacted Customer service and they were very helpful. Unfortunately, they couldn't help enough and so I've had to take a refund and rebook. Not ideal, but such is life.
Basically the AY booking engine allows any flight that is oversold in I to be put into Y without proper warning during the booking process. The lady at CS said they were aware of the problem and were constantly badgering their providers for a fix. (This should not be difficult). She apologised profusely.
Thanks again for your help JnsV. Your quick explanation saved me a fair bit of international call time!
So a warning for all. Make 100% sure that all your bookings are in a Business Fare bucket before you press buy!