When to start feeding tomatoes?

yes Once you pot them into the baskets they will have new compost (and feed) to get their roots into

The standard advice on all liquid tomato feeds which I have seen, is to start feeding once the first truss has set.  This means when the first 'branch' of flowers have gone over and you can see the tiny little tomatoes where the flowers were.

Follow the instructions on the bottle - don't over feed.

what happens if you do overfeed?

as a novice of just over 5 wks now i got my miracle gro soluble plant food delivered and promptly fed all my tomato seedlings with it when they were only 1" high and suprise suprise ---they all died diddy bones --lesson learned,but as you say different advice in different places hence you can only make your mind up when you have the mistakes --and successes from other people to go by,plus if i had taken the time to read the outer bucket i would have known not to do it     

Thanks all! I will hold off the feeding. 

Heated or not?

Heated, anytime. Not heated, mine will go in next month.

I was going to put mine in unheated polytunnel at the beginning of May. Is that OK if I can't cover them with fleece at night or should I leave them a bit longer?

I've put them outside today to start hardening them off.

They didn't like the first taste of the outside world! When I came home some of the leaves were very wilted, poor things, so it was probably too cold for them yesterday.

I think I rescued them in time so hopefully they haven't been set back too much!