Well I had a look today and stripped the airbox off to have a peek.
I do admit to having water ingress into the coweling but don't think it's water in the airbox as it's slightly oily but dry looking.
Instead I had a look at the throttle plates inside the carbs and found that when operating the throttle lever one of the plates lags behind the others slightly, so I uncoupled the linkage that connects the 4 Carbs. After opening and closing all of the Carbs independantly I found that the top two snapped shut under spring pressure but the one that lagged behind was binding slightly and would not close fully under it's own steam, also found the lead carb doing the same.
I think it could be coincidence about the water and this problem. I'll book an appointment with the guy that looked at it last time.