I sow small seeds, mostly flowers in trays and large seeds, eg courgettes, runner beans in pots. I often put a stick such as short piece of bamboo in the middle of the trays and grow different things in each half. I enjoy pricking out. Some plants don't transplant well, such as carrots, beetroot, so if you want to get an early start then do as figrat does and use lengths of guttering in the green house to be slid into a hollowed length in veg garden later. I have 3 small electric propogaters and a greenhouse but I haven't sown a lot yet as it's been cold and when the plants are up and pricked out into pots then take up a lot of room in the greenhouse and I order plug plants too which will be arriving the end of March.
I've sown 3 sorts of tomato, French marigolds, ageratums, penstemons, which are up now but too small to prick out. Will be sowing more, such as courgettes, squash, cucumbers, runner beans, later. Sown broad beans and peas outside, not up yet, don't know if they will be owing to awful weather.