"........So, when should I sow the sweet pea seeds? I have a greenhouse
with a small heater which keeps it to about 5 degrees. If I plant the seeds
directly into the soil will they grow fast, or is it better to start them
off earlier in the greenhouse?...."

I would sow the sweet peas now keeping them cool, once they have made 3 or 4
leaves pinch the tops out to make them bush and grow them on cold.
When you plant them out plant them about a foot apart, with a runner bean
between each plant, this will give you quite a thick cover and will also
give you a reasonable crop of beans.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk