New allotment

Hi all,
I've taken on a disused allotment its never really been worked its covered on couch grass which I'm slowly digging out of my heavy clay soil.
My plan was clear the plot, dig out roots, dig in compost (about 3 tons has now been dropped off) I only seem to have about a spade and half depth of soil then hit yellow clay. Hopefully this will improve drainage . I need to build paths and beds put in a shed, greenhouse. I also want to add some manure and maybe grow some green manure over winter, then come spring have an allotment ready to go.

I'm now doubting my plan and some are saying don't dig in compost just spread on top and wait until Autum for muck spreading.
Any advice gratefully received.

one idea is to put a small wooden border around your plot say about 10cms or 15 cms tall and then bun in teh amnure , this will let u get 10cms above the rest of the area around u and increase the depth of your plot....