It seems the only thing retarded is me. I should have checked the simple things first. It might be fuel. The photo's below show new fuel and old fuel samples (I don't think the pictures highlight the difference as well as true vision). The old fuel was very cloudy even and though it has a fuel filter in-line I don't think a filter would stop contaminated fuel but is there for debris only. I have drained the tank and carburettor and it now starts. I'll try it again in a few hours just in case it is a fluke. I know bad fuel can cause starting problems but could bad fuel cause backfire through carburettor?

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