manureing the allotment

You can put a layer down now, leave the worms to pull it down over the winter, then dig in any remaining in the spring.

You want to make sure any perennial weeds are removed first, and also loosen the top layer with a fork if it is very compacted.

I would put it on now without forking. The worms will pull it down into the soil over winter leaving you an easy job in the spring. A light forking and any weeds should pull up a lot easier.