|Thanks for all the responses. |
After several attempts I managed to update the M6300 Bios from A11 to Latest A15. (Note that the M6300 Bios update from Dell does not work under Win7 - apparently a now known issue. The update only works on earlier OS up to Vista. I was able to boot from my old WinXP drive that I had fully replaced during upgrade to 7 pro and upgrade the Bios then reinstall the Win7 Disk. After all that the new Bios still had no way to disable the Power supply warning or any way to ignore it.
It appears that the only answer is to buy another genuine Dell 130 watt power supply and ignore the many 'Aftermarket' 130-150W units that claim to be valid Dell 130w replacements. Or continue to boot on battery alone then plug power back in.