There is a long running debate on when to sow sweet peas. In fact on our very own site we suggest sowing them both before Christmas - see Sarah Raven's video:
and after Christmas (Jan to May) - see the following project:
Personally I favour after Christmas. I've had good results from spring sowings. It saves you the hassle of looking after them over winter. Giving them nice fertile soil (dig in lots of organic matter) and watering a lot both help enormously.
Also, the earlier you sow, the earlier they'll flower and ultimately die. Ideally you'd probably sow some early and some late, to have sweet peas for as long as possible.
I hope you get lots of bunches of blooms from yours,