Success!
After removing the sump pan and thoroughly cleaning it I secured it to the bench (bottom side up) with screws through the bolt holes, I drilled a pilot all the way through the plug .
Then after failing to remove the plug manually with a bolt remover,cordless impact driver and a 750watt wrist snapper high torque drill . My last resort was the winner (only just) a larger bolt remover with a 10mm socket driven by an air impact wrench at 120psi.
Make sure the pan is solidly secured if you ever do this
The issue with the first three attempts was purchase on the remover as each method slipped once the remover truly bit into the plug. But having said that I'm not sure I or the driver/drills could have exerted enough force to budge the plug.
Thanks for all the input, hope this is of help if anyone else gets themselves in this predicament.
TTFN.