Tomato plants need as much light as possible, so keep them in a conservatory or on a sunny windowsill (but don’t leave them behind the curtains).
On warm, sunny days you can put them outside, but don’t forget to bring them inside at night. Ideal temperatures at this stage are 15°C at night and 20°C minimum during the day.
Your plants may need to stay in their pots for several weeks until the weather is suitable for planting. Space them so the leaves don’t overlap. They may need liquid feeding, especially if you have used seed compost to pot them up (it is usually lower in nutrients than potting compost). Use a proprietary high potash tomato feed made up according to the label recommendations. Typically they will need feeding from about 4th true leaf stage, but if the bottom leaves start to turn yellow or purple, this can be a sign that they are running out of nutrients (purplish tints can also be due to low temperature).
Did You Know?
British Tomato growers use an army of over 100 million beneficial predatory bugs to help balance glasshouse ecosystems where pests are controlled by predators and so are present at a level below those causing economic damage and thus reducing, to virtually zero, the need to use any pesticides which if they are will be primarily biological in origin.