Here, we feed our citrus trees every 3 months. Yes, you'd probably burn the leaves if you added citrus fert right now after you've already put in some other plant food. Wait a month or so. You're right, a 5yr. old citrus plant should be 3 to 4ft. tall by now. How about posting a picture? They need bright sun, warmth and lots of water with good drainage, as well as regular, specific plant food. Also, a tree you grow from a seed will not give you the same type of oranges that you got the seed from. Oranges are always grafted. So what you get if you do eventually grow it to bearing size might be really small, sour oranges.
You're growing a lot of tropical stuff, over there in the UK. Are you growing them in a greenhouse? Do you have some supplemental lighting for winter?
