When I had the allotment I used to sow Fodder Radish
Seed immediately after digging up my Potato Crop. The soil was warm enough for it to germinate, it kept the weed seedlings down. That was September.
It grew well, then I dug it in during November. It rotted
down during the winter. Gave the soil humus ready for
spring sowing and conserving moisture the following summer.
I dont know any other way.
Fodder Radish Seed is available from Central Wool Growers
next to Morrisons at Stamford. I bought a large amount, still have some left in the cupboard.
Will continue this routine in my raised beds this year.
Will dig in the Spinach plants a s a p. as they have grown too big. There isnt the space as there was on the allotment. Still miss it but not the vandalism which upset me.

I thought you left it over the winter to protect the soil and dug it in when the soil became workable in spring? be interested to say what allotment gardeners say.