I wouldn't. Just put it in a much lighter position. If you want to stop it later it would be as well to wait a bit.

I agree with steragram. perhaps you have started them off too soon. if you can set another set of seeds off in a week or so.

Come to think of it your other seedlings are looking a bit drawn and leggy too - a symptom of too much warmth in a low light.

I have to put them on my dining table as I don't have heat in my greenhouse. It is a south facing room but its obviously still not light enough. Not sure what else I can do other than get heat in greenhouse.

Just delay sowing for a while. You could use an electric propagator (or two!)in the house, move to a windowsill if you can when the all seeds have germinated (not in strong sunshine at first though)and prick out and transfer to the greenhouse when they have several pairs of leaves.Then use the propagator for the next batch. If you do the hardiest ones first they should be safe in an unheated greenhouse as long as you stand by with fleece if necessary. If you want slow growers like fibrous rooted begonias you might be better with large plug plants later.

Many later sowings pretty much catch up with earlier ones anyway, and will be stronger than ones that start off drawn and spindly which often have problems standing up straight!