i presume you mean the height of the tomato plants and not the seeds !!!
When you get 4 or 5 trusses of fruit set, pinch out the growing point at the top of the plant, this will allow the fruit to get larger and ripen before the end of summer.
Unless you,re growing bush tomatoes, don,t forget to pinch out any sideshoots as well, these are the shoots that grow out of the leaf axils. hope this helps you

No it is the seed, is it too much heat (looks like cress)

I assume they are getting a bit leggy and you aren't growing them in a greenhouse?
You can transplant them into a deep pot up to even the 2nd pair of leaves...that'll make them less leggy.
I haven't sown mine yet...plenty of time to start again if you can't save them.
Sowing mid March means i get tomatoes early July onwards......

Hi Thomas you have started your tomato seeds to soon and the seedlings are becoming leggy as they look for the light. Having no idea where you are in the UK I'd say if in the south then leave for another couple of weeks, north not until April.

Thanks everyone. I am in Inverness

Then definitely far too early Thomas try again in early April. I know the temperatures rose to a dizzy 10˚ last week but it is still winter as can be seen by the snow over the past three days.