To quote Mark Twain - "In the Spring I have counted 136 different kinds of weather in 24 hours." Not so different in recent days from shivering sub-zero to basking in 18oC warm sunshine. So don't be tempted to start your seeds off too early unless you can move them inside or cover them with cloches or fleece when colder weather threatens.
Though what to sow now? If your ground is warm then outdoors sow beetroot, carrots, chard, summer cauliflower, kohl rabi, lettuce, leeks, radish, turnip, spring onions, peas and perpetual spinach. Flowers include calendula, oxalis and clarkia.
Prepare a nursery seedbed for brassicas to transplant in June and July such as sprouting broccoli, cauliflowers, cabbage and kale.
Indoors; sow marrows, courgettes, pumpkins and squash, sweet peppers, tomatoes, cucumber, aubergines, celery, celeriac, salad leaves and globe artichokes. Start indoors half-hardy annuals nemesia, rudbeckia, cosmos and french marigold, sunflowers and nasturtiums for planting out after the last frost.
If the weather is unkind, don't worry too much, any varieties sown later in the month will soon catch up