I always baby old 110v leads from car boots. Have more than I ever need but the old 110 stuff tends to have good quality coverings.2,5 3 core is ideal its whether u use plastic or rubber . depends on what you want to spend plastics ok rubber more durable and flexible
For general garden work, electric saw, hedge trimmers just general use really about 20 meters would probably do to be honest I'm going to invest in cordless as soon as I can to be honest.What's the extension lead for and what length?