Heavy clay soil at my allotment

[Prepper1]

Hi there guys, I managed to get half an allotment off my local council which measures 12 meters by about 6 meters workable area.
unfortunately the heavy clay soil is holding on to the water and it looks like the Somme at the moment. I've put up a 12 meter long by 3 meter wide poly tunnel to help keep the rain, slugs, cats and birds off next years crops.
I researched how to improve clay soil on the internet and it suggested adding sawdust and horse manure in large quantities to improve the soil and help drainage possibly sand as well.
As well as that I was looking at getting a manual corkscrew post hole drill to drill many 3 foot deep holes all over the place to drain the water.
Does that sound like it'd work? it does to me but I'm a complete newbie in the art of drainage and soil improvement so any advice would be greatfully received.