Hi Ben, the cold weather certainly makes for dramatically different conditions to previous winters. Worms are less busy near the soil surface – I think quite a few are hibernating. On the other hand, the frost is getting into all lumps of manure and compost so that when it thaws out, they will crumble apart when knocked with a rake. In this way, the manure or compost becomes a lovely dark, crumbly topsoil for planting into, and worms will come up for it in spring as plants are rooting into it.
I have just finished spreading cow manure on a bed where parsnips had to be levered out because they had grown so long in the undug clay. This levering loosened the soil more than I like for all vegetables except potatoes, which do grow well in artificially loosened soil. So I shall be planting potatoes into this soil, under the manure which acts as a fertile mulch on top and can be used to earth up (I should say ‘manure up’) the potato plants as they grow next spring.
Then after harvesting the potatoes (first and second earlies) in late June and July, I shall rake level all the remaining surface manure on top of the bed, before planting autumn salads such as endives, radicchios and oriental leaves, which thrive in the rich soil which usually, by then, has seen most of the manure taken in by worms during spring and summer.
Re the polythene, it may do some good if you have weeds to kill but otherwise I see little benefit, and it sometimes helps slugs to breed.
In other words, I never incorporate manure or compost. Vegetable plants love to root through them on the surface, and into the undisturbed but well structured soil underneath. Last year my potatoes were wonderful, grown as described above, and easier to harvest because of the tubers being closer to surface than normal (less sticky clay to deal with), as long as one makes sure to push compost around their stems in May and June to stop any tubers going green.