Thanks for the replies.

Had a very quick chat with one of the guys from pre travel support who said that the IAH-RTB flight was not showing on their system at all. He said the system would usually show the flights and either be available or unavailable for a redemption booking. He said he had been advised that this meant that BMI did not have an agreement with the operating carrier to book redemptions on these flights. He's passed on my points to Donnington...