Did you say you had a greenhouse or not? even so when i did not have one i would start off seeds on the sill and when they were growing climatise them by removing the lids and dropping the humidity they are used to. let them grow on and when big enough and strong enough , then move them either into a greenhouse or why cant you pop them into the poly tunnel at night for protection. I dont have a heated propogator just a parafin heater which i use only occasionaly when desperate. what is it you are growing from seed? sorry probably not much help :)

Most reasonable propagators are ridiculously priced. I recently saw the one I used priced at 250 euros, for a 50 watt base with a plastic surround and lid! Luckily mine only cost me about £70 ten years ago (and I was still conned!)
I'd recommend you get one of the heated trays from Two Wests and Elliotts, and build your own cover for it with a wooden frame and polythene (to use in your tunnel) or even with a glass and wood structure. Alternatively you can still find the solid tubular heaters which give 150 watts or so that you can fix just below a metal staging tray and will give far more heat to bring on seedlings. You can save money on the electricity by fixing a thermostat to this, but these are also quite costly items. I tend to use cheap ones designed for interior use but I wouldn't recommend this as it's not safe in a greenhouse setting, so use a thermostatic control from Two Wests and Elliott. Although quite pricey, these things will last for almost ever so it's a good investment.
Once you have heat, you can germinate seeds and bring on cuttings in January and February and many small plants will then survive outside the propagator if you harden them off, or cover them up on frosty nights.
I think a propagator is the best investment you can possibly make and I wonder at the stupidity of those who heat a whole greenhouse to achieve the same effect.
One word of caution, look at the power rating of propagators and remember that in the cold of a polytunnel or greenhouse, a 25 watt or 50 watt one will have great difficulty in raising the temperature sufficiently to germinate lots of seeds needing fairly high temperatures.
Also, with the lid closed, when the sun comes out, the temperatures soar. I've lost far more tiny seedlings to 'cooking' than I have to frost. So always leave the lid off on a sunny day.
Good propagating!

The idea of having a heated propagator is to do "what it says on the tin" and that is to provide heat so that your seeds can germinate, if it's got a thermostat that's even better, but it's not a necessity. You can buy reasonably priced one's , I brought mine for £45 three years ago, B&Q. When the seeds germinate, you must get them out and as long as they are not subjected to very cold temperatures, and protected from cold winds, then there shouldnt be a problem. I'm sure you know most of this anyway, but......
I have a heated greenhouse, heated by an electric heater, which is set on frost free. I also have a small paraffin heater £20, this is on most of the time. It might be a good investment to buy one of these and keep it in one of your tunnel or grow house I also propagate in the greenhouse outside of the propagator, I don't think I'm stupid doing this as I do it to get early shows, have a look at my blog. You may also want to look at Bulbaholics blog, he propagates in his front porch with no artificial heat.

Thank you so much I really appreciate your answers they have been so helpful will just buy a seed tray for window sill due to price and take it from there
I do find this site after several years even more valuable with such knowledge...

I was surprised at the price of propagators. We unearthed one while tidying up. GJ wanted to dispose of as rubbish and I blocked the idea after checking on current prices. I have to prove its worth by using it or passing it on!! It is currently in use, but nothing sprouted yet.

Something will sprout soon Dorjac I'm sure. What seeds have you sown ? Don't forget with some seeds, it might need a little more light. Very much in short supply at the moment.

I sowed some seeds sent by a fellow GoYer from a plant that grows to a great height over like 7 years then flowers, sets seed and dies! The pod is huge and crammed with flat seeds designed to fly through the air. As GJ is on a throw it out kick, I had to plant something. There's lots more in the pod. Not much light, alas, but lots more seeds for the save the propagator campaign.