This will be temperature related then. A bit more info from you would help but I guess they are inside? They need to be put somewhere cooler but still in good light.

they are in a cool bedroom window, light not great, I cannot put them in greenhouse, it's too cold, could I nip out the tips? make a cold frame from some polystyrene boxes and polycarbonate sheet and put in a sheltered spot outside?

You will be surprised how hardy they are. Surprisingly, they can take a few degrees of frost though normally only once hardened off first.

Have you nipped them back yet?

I have just done a quick search and you should get all you need to know from here: http://www.reneesgarden.com/articles/swp-transplant-tips.html

Good luck with them David.

I'd pinch them back too are they still in seed trays or potted off

My autumn-sown seedlings are all out in a cold frame and doing fine ... I pinched them right back as was also concerned they were growing too much too soon, and they've started to get bushier. My cold frame is a very cheap plexiglass type affair against a warm side wall. If you haven't already done so, I would pinch the tips out.