Join the club. My tomatoes are so small this year it's actually embarassing I've not changed anything that I usually do, but they just don't seem interested in growing. i have the same set up as you, an unheated greenhouse, and nothing seems to have really happened. So sorry can't help but at least you know you're not alone.
Mine are all slow too... I was looking at my tomato plants this morning thinking how ickle they look... I have 1 courgette plant that is establishing itself well but the other squash are barely at the first 2 leaf stage. As are my aubergines, peppers and chillies.
I am hoping the weather will warm up and they will jump ahead in the next few weeks.
Ann Pan
"Some days you're the dog,
some days you're the lamp-post"
I've just been to the garden centre to buy tomatoes and came back with orange and black peppers (never ever heard of black ones before!) and 3 aubergines as well! Anyway I had to as I was on hols so couldn't grow toms from seed this year. Way behind on everything else as they've only just been sown in the last few days.
Perhaps putting tomato plants in an unheated greenhouse too early is what is checking the growth. Wouldn't it be better to grow them inside in the warm for a bit then put in the greenhouse later on when the weather is warmer at night?
My toms are looking a bit small and sad too!!
Last year a friend gave me some plants that she had sewn late, and they kept fruiting until the first frost killed them.
Stay at home Mummy to Orin 8, Trixie 6 and Temogen 4 . Also three Chickens Dottie, Poppy and Dr Mumbo. Three cats called Flossie and Pickle and Lexi.
Phew, there I was thinking it was just me. Mine are all really small as well.
A friend gave me a couple of toms that are a lot bigger than mine. She had planted them very early in her conservatory as she was going on holiday so they were a lot earlier than they should have been.
My tomatoes are small too, though they seem to have picked up the pace a little over the last week or so.
Maybe it's just because I stopped checking them every half hour.
Mine were so pathetic I decided to sew some more, much more successful but ofcourse behind...so what do I do with 75 embarrasingly small tomato plants, I can't exactly give them away when teh agrden centre ones are 2ft now!
My tomatoes are only 1 inch tall but I already lost one lot of seedlings (about 40 tomato plants) to slugs in March so I really don't have that many seeds left
My spring onions look pathetic too... and they are in the Polytunnel too...
Oh Bums
Ann Pan
"Some days you're the dog,
some days you're the lamp-post"
It looks great Susie but I'm wondering what the marigolds are doing in there? I tried growing them last year amongst carrots as they are supposed to help repel the root fly (they didn't) but how do they help toms - or is it to attract insects for polination?
Planting marigolds with tomatoes is a standard way of keeping white / green fly off the tomatoes. Remember my grandad always had loads in his greenhouse and I've always done the same.
I've never had greenfly on my toms but then they've always been grown outdoors. I do have some marigolds coming along so I was just wondering what to plant them with this year. They are too teeny yet to put under the currants which have a lot of aphids on them, ditto the broad beans!
I've not changed anything that I usually do, but they just don't seem interested in growing. i have the same set up as you, an unheated greenhouse, and nothing seems to have really happened. So sorry can't help but at least you know you're not alone. 
(thanks)
but I already lost one lot of seedlings (about 40 tomato plants) to slugs in March so I really don't have that many seeds left 
