I've started using an idea I saw in a magazine - you sow 5-7 peas in an empty fruit juice carton (taking off both narrow sides to make a bottomless pot that can be planted whole).
5 of these 'bricks' will fit into a standard gravel tray - the equivalent of 1.5 metres of guttering - a lot easier to manage and deeper rooting for stronger seedlings too...
I've since found the central section of 6L milk cartons will work the same way at four to the tray - shorter row but bigger and better for climbing beans.
Cheers.