Hayward H200 pool heater wont start

[vwgolfs]

Hi Guys I am new on here so would like to say hello to everyone on here. I hope some one can help, I have a Hayward H200 gas pool heater, it worked fine all last summer without a problem, it was drained at the end of the summer and winterised. I have just reconnected everything and fired the pump up and it works fine pushing water through the gas heater, the problem is but when I turn the electric on the pool heater the green light doesn't come on, it still doesn't come on even when I switch it to the pool setting, the bizarre thing is when I turned my water pump off the heater fired up straight away! I immediately turned the pool pump on but when I did that the heater turned it's self off and the green light went out, so the heater only works when there is no water being pumped through it!! I would appreciate any help in trying to resolve this problem, many thanks