And the outcome...

Check-in at Austin resulted in a boarding pass from AUS to DFW for me with the bag checked in straight to LHR.

Check-in lady said she couldn't access the seating for the second part of the flight from her system and suggested I go straight to the gate on arrival at Dallas and the BA staff would check me in for the second leg and issue a boarding pass.

Worked like clock-work. BA gate staff were at the gate by the time I arrived (2.5 hours before departure) and immediately checked me in to seat 64K without prompting.

They said that everyone transferring from a local flight needed to visit them at the desk to have their documents checked, collect a boarding pass and have their luggage authorised to be transferred on to the flight with them.

Flight left slightly late (due to head winds on the way over) and arrived in LHR early (due to tail winds on the way back).

Thanks for the advice. In retrospect I should have trusted them to sort it out but always worth a double check for peace of mind!

Matt