Posted: 06/03/2014 at 15:50
djjj, it depends what you are planting; I have for years just germinated seeds in seed trays with plastic 'lids' on south and east facing window sills. In the main it has worked well. The small plants then went into my unheated conservatory. But this year because I wanted to speed things up and grow a few extra for the flooded gardens, I treated myself to a heated propagator (£30 approx). It has worked v. well so far.
I think your problem would be not so much the germination, but what to do with them when they are 2 or 3 cm high. They will start to bend towards the light source and could end up becoming 'leggy' or etiolated. If you had a green house, you could pot them on into little pots and keep them in there and then they would receive more light, but there is a danger they may get too cold if we have a frosty spell.
If you are really dedicated, once they have been potted on and are in a plastic greenhouse, put all the little pots in seed trays so that they are easy to move and if the weather forecast is really bad, whip them all into the house overnight.
Hope this helps