first time for leeks and onions (from seed)

The heat could be your problem.

John's article on onions from seed specifically mentions the fact they don't need too much:

http://www.allotment-garden.org/vegetable/onion-shallot/grow-onion-seed.php

IMO if you dont have a lighting system for growing them, it is a waste of time using any heat for onions or leeks. When sowing them early, i sow them in modules in a bright place indoors in modules. Sowing them into modules means the critical stage at which you need to transplant them is bypassed. They have plenty of room for a while.

this is a good question because i was going to set some onion seed this week but from what's said i think i mite hold back a bit

i take your point about artificial lighting the thing is if i leave sowing my onion seed  till after February will the onions catch up i know we're only talking about a few weeks later

but the nights are beginning to get a bit lighter and perhaps i said perhaps the weather mite improve