Just found this thread, and glad to see lots of fellow radio drama fans
For current radio drama from the BBC there is a fantastic free program called Radio Downloader which will automatically download the output from the BBC iPlayer. It can be scheduled for series or individual shows. I use the BBC weekly schedules to see what's coming up and then subscribe to those shows that I want to listen to later. I transfer them to an SD card in an FM transmitter to listen to when driving to work. The program can be found here:
http://www.nerdoftheherd.com/tools/radiodld/