Posted: 05/02/2018 at 11:18
I think Iain has nailed the problem. As he says try nipping out the growing tips to promote sideshoots and give them as much light as you can.
Sweet peas are very cold tolerant and don't need any cosseting during the winter other than protection from extreme wet and cold winds. Basically a well ventilated cold greenhouse or cold frame is all they need. As the days lengthen acclimatize them to being outside to get them ready for planting around mid March depending on your location.