Tomatoes in mini greenhouse

I've got two tomatoes growing in mini greenhouse, but they've now reached the top of the greenhouse - should I cut the tops off?

I'm really not sure what to do! 

Fantastic - Thank You!

I haven't done anything to them at all, have green toms at mo.

How do I pinch out shoots from leaf axels.  That is, what should I be aiming to be left with?

From what I hear, we all seem to have green toms at the moment...!

Come on heat!