Well, after connecting to the internal serial console and fiddling around with the U-Boot loader for an hour I managed to unbrick my router and upload the latest RBR50-V2.7.1.60 firmware. Yay!!! 
The router had lost almost all of its configuration state - including it's MAC addresses, hardware id, model id, etc.
I needed to set these to back sensible values be able to bring up the TFTP loader and load/flash an image. (luckily the error message saying that the image file didn't macth the hardware ID told me what values I needed to configure!)
Then it took a couple of reboots to come up happy. Still had a bogus SSID and unknown passphrase (so the iPhone app was useless) - but at least I could log in on the LAN port and manually set thing up.
Now it seems to be happy I will see if I can find my configuration backup file 😉 (you know, the one you always make but have never tried restoring!)
Time for a beer!
