Don't want the .m4a file

[James Scholes]

I would suggest writing a shell script which is responsible for
converting the M4A to MP3 and deleting the original, in the right order.
 Advantages of this approach are that you don't leave it up to
get_iplayer to run the commands one after the other, and that you can
check within the script that FFmpeg hasn't exited with an error code
meaning you delete the original by accident.
-- 
James Scholes
http://twitter.com/JamesScholes