Then I realized I had had another senior moment ( well I do clock up three score years and ten this month ) and forgotten the difference in density.
I had reasoned that as both blades were 2mm thick and about the same area they would probably weigh about the same but the new blade was about 8 gms heavier.
By the time I had hacked enough off of it to get the robot in weight using a dremel and diamond disc the whole blade had gone blue suggesting I had annealed it so I had to reheat it all up and immerse it in oil to re-harden it.
Here is the final blade which only time will tell if it is and improvement.