[Allan Morris]

I am looking for advice on heating a 15 ft by 10ft polly tunnel mainly for starting young plants

[fatcat1955]

The cost would far outweigh the benefit's......unless you are growing commercially. Wait until the sowing time as printed on the seed packet and you should not need to heat.  If you want to start thing's a little bit earlier make another tunnel inside the existing one and sow your seeds in there.

[muckshifter]

I have a 3 kw thermostaticly  controlled propane heater in my greenhouse and used it for around 16 weeks last winter at a cost of around ninety pounds, for that outlay my greenhouse was at a constant 55 degrees regardless of outside temperature
I did shop round for propane as exchange bottle prices vary by around 8 pounds around here.
Its maybe not cheap, but I get great deal of pleasure from my allotments and thats what makes it worthwhile to me.