Sunshine

I have spent the last two months at work, since I got my greenhouse, looking out of the window and everytime I saw the sun shining thinking 'oh good, the plants in the greenhouse will be happy'.

It is only over the last week on holiday that I have remembered that, in actual fact, my garden doesn't get any sun in the winter months as the sun is too low  :roll:

I am considering heating the greenhouse next winter, but my question is, is this worth it without sun?  Do vegetable plants need the sun, or is it just the heat they need to grow?

Heating a greenhouse even if it only keeps it frost free is always going to give you earlier crops . What you have to weigh up is how much will it cost to heat against the value of the crop. I'd try extra insulation (bubblewrap) and see how much difference that makes be worth putting that in place now . Don't forget first thing Max Min thermometer and see what the current temp is

You need to insulate the roof as the heat will rise and escape through the glass Whilst the bubble wrap will block some light  the heat gain is more valuable IMHO.