Tomatoes sown in January may well be ready for potting on now.
We left them in modular pots both to save space in the greenhouse and to slightly stress them into flowering. Once they are flowering they can go into grow bags or pots.
I use builder's buckets with holes drilled in the bottom as they only cost £1 each and hold water and nutrients better than grow bags. Fill them with home-made, multipurpose or specialist grow-bag compost. Tomatoes are thirsty and greedy, so you will need to feed them once a week once the flowers have turned to fruit.
Keep them inside until all risk of frost has passed. In warm wet weather keep an eye out for tomato blight which turns the leaves and then stems black and destroys both tomato and potatoes. If you get blight all is not lost - pick off all the leaves and destroy them, give the plants a good feed and they may well recover. (Spraying with copper sulphate or mancozeb is an option but I have managed with just feeding in the past)
Tomatoes in modules waiting to flower