So by way of an update.
Arrived at LHR and of course no sign of the bag. Went straight to the AA baggage counter and related the sorry tale to them and filed a baggage report.
Whilst at the desk they checked and found a report on the bag advising Avianca had put the bag on their daily direct overnight from BOG to LHR and it would be couriered to me either later this evening or tomorrow as that flight lands early afternoon LDN time.
Will be following up with AA on the shocking conduct of their agents at PTY. That sort of shady crap needs to be stomped on hard.