Well, that worked pretty well, thank you burakkucat
I followed your advice, used ssh via PuTTY, invoked the 'zycli save_default clean' command, got the == Clean ROMD. == message and then entered 'hexdump -C /dev/mtdblock3' to verify the ROMD was clear, then hard reset the device with a pointy thing and hey presto! I have stock firmware on my machine
The maintenance menu now includes Firmware Upgrade and Remote Management but no TR-069 or TR-064 control, dunno if ZyXEL have removed them in the firmware...
I have used the Firmware Upgrade to successfully install the latest firmware V5.11(AAVF.3)C0
I'm well pleased as I got the VMG3925-B10B for a song on ebay, it's almost new and it's a lovely little router
Thanks evryone for all your help, special thanks to burakkucat