The posting I made above yields a few clues.
Chiefly, the single 0845 number directs calls to the different offices. With this in mind, the 0845 number probably puts calls through to geographic numbers at AvMA's offices and thus these numbers will bypass the mechanism on the 0845 number.
Calls are dealt with in Manchester and Croydon. Steven Liddiard is based in Manchester and his direct number is 0161 819 6858.
The question is, is Mr Liddiard's direct dial number a clue to those which belong to the Manchester office?
The number 0161 819 6860 is answered by a recording identifying itself as Key Travel as do some other numbers above 6860. Key Travel's
contact page gives telephone 0161 819 8900 and fax 0161 819 6861. This perhaps rules out numbers 6860 and above as belonging to AvMA.