Years of experience and watching other allotmenteers show that, despite what the books say, seed-grown plants seem to do better on many soils - including heavy ones, for which sets are often recommended. Seed-grown plants develop better root systems and are less prone to fungal disease.
Old-fashioned gardeners (including Melvin, whose veg are show-class) sow them outdoors in January. This year, however, I think even the most die-hard advocates of outdoor sowing may think again. Others sow in trays then prick out to cells, keeping the plants in a cool greenhouse and hardening off before planting out in March.
The first time I tried growing from seed I gave up and bought sets - those tiny weedy things were never going to come to anything! My mistake, "plant them out when they look like little bits of green cotton" I was later told. Little bits of green cotton certainly described my seedlings - I left them in for spring onions and it became plain, later in summer, that if I had persevered with them I would have got a decent crop - Bill's seed grown onions were psectacular last year, while my set-grown ones were miserable. Lesson learned!