Question about my tomato plants

I have 6 tomato plants in my greenhouse.  They're all cherry tomatoes.

They're all growing tall - about 3 to 4 feet - but don't seem to be producing many trusses. 

I understand that I should pick the tops out when 'they reach the greenhouse roof and have 4 or 5 trusses'.   Mine are starting to bend, having reached the roof, but have only 2 or at most 3 trusses each.   Each truss has only a couple of flowers on it.   The plants look healthy but slight on the frail side.  I've been feeding them.

Advice please.  Should I let them keep going up in the hopes that they'll produce more trusses or nip the tops out?

On the other hand the cucumbers I'm growing on the other side of the green house are amazing.  I can almost see them grow and the fruit they're starting to produce means I could live on cucumbers and nothing else for weeks if I wanted to!

Have you been picking the extra growth out from between the main stem and each leaf stalk.?

Yes, I have been removing side shoots.

I usually grow from seed but this year due to a number of reasons I bought plants from the garden centre as I was behind schedule. 

I can't remember what varieties they were but I do know I went for heavy croppers - which they certainly aren't looking like at the moment - and I got two different varieties, 3 of each. 

This is probably unconventional but since our tomatoes reached the top of the mini-greenhouse with only three trusses set (the fourth is trying to escape through the plastic) I've decided to let the next sideshoot that appears near the base of each plant grow and set a truss. Not much progress to report on yet...

Interesting experiment, I'll be interested in what you discover, please report back.

It's nothing new you are just growing them as a double cordon, see this thread and pic 

http://chat.allotment-garden.org/index.php?topic=80277.msg899632#msg899632