Decided that the weather was nice to have a look at the burner again, put endoscope in and as usual brain not thinking as there was the "Fuel pipe" across the base.
Took out the plug and next idea get one of the bolts of same thread, yup not got one, had one M10, think the thread could be M12 pushed gently and no crack I was expecting if rust, it moved gently, back to original thought sludge, put about a litre of petrol in to soak over night, will give it a swirl out tomorrow before using a bit of the brake cleaning fluid.