Just got a reply from RFDigital
"The RFD21815 and RFD21807 is not suitable to load code wirelessly to an Arduino. The parts you have are best for sending serial data back and forth like a wireless pipe."

So it looks like it's a hardware issue. Hope this helps others avoid the same problem

I've connected to my Uno using RFDigital's wireless inventors shield and their usb rf dongle.
This configuration successfully echos bytes sent to the Uno from MLP.

Next I set up the Arduino Upload Monitor and tried uploading a small tweak to the test program.
The following errors were given ...