Buy an aluminium heat mat and if you have a spare room start them off in there. They come in all sizes.
Once germinated, stick them in the grenhouse.

What are your seeds?
I find french marigold for example do fine in a cold gh with a plastic propagator lid on top......get them with air vents, it prevents condensation.
some seeds like toms, peppers etc. do need the heat though.

be aware too if you start them too early you can't put them out until danger of frost is passed, where I live thats usually late may, they need hardening off as well .....out in the day and either covered or in at night.

No spare room and the greenhouse is down bottom of garden so cant put electric to it, i was thinking of a green house heater which uses paraffin but not sure what temp it will maintain ( green house is a 8 by 6 foot)
As for what sort of seeds I have about 40 packs but most say seed mid march with some sort of heat

window sill and once germinated move them down to green house.
I regularly start mine off later and they soon catch up and as i'm NE they dont get planted out till June.

A lot of the bedding plants will be okay they just take a bit longer to germinate without the heat but soon catch up, in fact sometimes they can be stronger as heat forces them and theres always the possibility of them getting too leggy.......As has already been stated you can start them off in the windowledges and then pop them into your g'house....

Thanks for all the replies I have now invested in a new paraffin heater which should arrive sometime next week so I am going to hold off until it arrives then i will test it over a few days before starting to sow my seeds I will keep you informed on my progress regarding the heater