tomatoes not flowering

If they are tall as you want them to be nip out the tips and reduce all the side shoots to about 5 only. dont feed until the flowers have formed and set their first fruits.

Were they grown from a seed packet or from seed saved from fruit/friends? Are they in enough light?

Tomato plants should remain in their (small) pots until the first flowers form and start to set before being planted out, otherwise you commonly lose the first truss. It sounds like your tomatoes are being treated too well, with too much feed and water as they are just making leafy growth. They will flower soon but you may find they only produce fruit on the end of the shoots.
As Volunteer suggests, poor light can be another reason why they aren't flowering normally so far, and I've noticed that tomatoes in our polytunnel which are shaded by other plants don't do as well as those in the light.