Much too soon to plant your sweet peas out Cookiegirl, I'd let them get taller than 2 inches before pinching out the growing tip.

I planted out a couple experimentally a few days ago and so far so good but I'm not holding my breath.

What are they growing in at the moment?
Whatever it is, get them off the windowsill and leave them outside. It is not too cold for them. Mine have been planted out over a month now.
The problem at the moment is soil condition, rather than cold. Unless your sweet peas have a very good root system, delay planting out until it dries out a bit. General rule about pinching out is to wait until there are are least 2 pairs of fully formed leaves, meaning the plant will be about 6 " tall.

thanks scrumpy...they are growing in trays and I measured them and they vary from 2 to 4 inches high...i will wait a bit longer if the weather today is anything to go by.