Heavy clay soil at my allotment

[NorthernRaider]

First get loads of horse muck, and sharp sand not smooth building sand, sawdust is OK so long as in general its from hardwood cuttings not softwood or pine. Now either hire a rotivator and dig it all in, or go the alternative route of building raised beds.

if you go raised beds you dont have a frainage problemn if you choose to work the lkand find the lowest spot on the plot and dog a decent hole at least 4 ft deep 18 inches wide and fill it with pebbles, rubble, gravel etc to act as a soak away.