Thank you KBIO, that's very useful, I did experience the steam extinguishing the burner flame on another model, it happened in the middle of the lake when the safety valve lifted with he steam being expelled through the funnel. It took a few moments to realise the flame had gone out, as the boat slowed down I turned for the shore, only for it to stop close enough to be reached with a fishing rod - so I will not be putting safety valve steam nor engine steam out the funnel unless the pipe is as you say exiting very close to the top of the funnel.
It occurs to me that I have not shown a photo of the union cock valve that has been causing me issues so here for completeness it is.
Also from the photos you will notice the gas tank sitting in water. Well this time I started with out the gas tank in water to see how well it worked. The boiler runs fine, it was quieter than on previous runs and as time went by the tank got colder, bearing in mind it takes 17/18 minutes to get this boiler up to temp. So after ten minutes I put the gas tank in the bath of warm water you see in the photo, well after just a few seconds the burner was louder and the flame visibly brighter. So there is a definite need to be warming the gas tank for this installation. The options are to use heat transfer (conduction) from the boilers burner via a metal sheet to the gas tank or to use the heat from the oil separator. However this second method would not supply heat to the tank in my case for about 20 mins so probably not an option. Anyway case proven for careful gas tank warming to maintain the pressure. One more thing to try/rule out or compare will be the performance with a disposable gas tank, since there is space for a c250 canister, will it be affected by chilling in the same way? it has a larger surface are for the gas to be produced from, will this be ok?