So the carbs have been cleaned out in the ultrasonic bath and re-fitted to the engine, new gaskets on the whole carb assembly have been fitted. I've also balanced the carbs with a friend's vacuum gauges so all should be perfect as far as carbs go.
Cranked the engine up this afternoon and to my delight it started with the first turn of the key (normally takes a good bit of cranking to get going from cold). It settled down to a nice idle and I let it warm up for a while.
However, once the engine was warm I had a go at revving it up a bit and sadly the lag still persists. There's a real delay in the revs picking up, especially at the top end. Also, once the engine has been revved for a while, it goes back to idling very roughly at 400-500rpm for a few minutes before returning to it's normal 800-900rpm, (which i found quite peculiar).
I'll be using the boat this weekend so I'll have an update on how it goes on the water. I've attached a link to a quick YouTube vid of the engine idling/revving, its a bit windy but you can hear the throttle delay.
P.S. The boat has a Yamaha 10micron external cartridge filter/water separator so I wouldn't have thought it could be dodgy fuel, but I could be wrong.