wonderful poly tunnel but sorry cant help with the heater.

My advice would be to give up any idea of heating the whole tunnel unless you have lots of money to burn. Instead, get a really good electric propagator or electric mats over which you can place small plastic tunnels or a home made cover out of polycarbonate.
You will succeed far more by heating a reduced area, both in propagating and growing on plants. If you want to overwinter tender plants, then it is better to have a more solid better insulated structure, especially something in polycarbonate or glass built against a south facing wall.
If you are planning to use the tunnel for vegetables rather than ornamentals, there are many crops which can be grown throughout the winter without additional heat. For ornamentals, you would save money by making another enclosure inside the tunnel, as Bob Flowerdew does with his famous bananas in East Anglia.