Time to sort out those rotten floor planks. Strategy: cut off the ends back to the next bearer, insert a well creosoted plywood section, then when the rest of the shed is finished over-cover all the floor with 18mm OSB3
I had an off-cut of ply that proved to be 1/8" too thick but it got pressed into service (literally with a sledge hammer!)
So, firstly cut a bit of floor to check the board thickness
Then fix a batten to act as a guide for the circular saw
Slice off the rotten bits
Cut plywood to size
Slaver creosote everywhere (including on me !)
Nail down the ply with the Paslode gun.
It ALMOST worked like that

Firstly the 13A socket in the shed wasn't working - quick bit of diagnosis found the socket itself was too corroded. Replace socket
Then the outer bearer was mainly ok but one end was soft - slice out another one on the Dominion saw bench
Other than that it went OK - the ply was a tight (1/8" interference!) fit but the sledge hammer sorted that!
Now at least there is a firm base to place the rest of the front framework that I'm replacing