[Mr Gazza]

The carb heater tap is a diverter valve, that is to say it always supplies the oil cooler in which ever orientation. As a general rule ball valves always have the handle pointing to the direction of flow. So yours is currently to the oil cooler only, when the handle points up the carb heater line, then the flow will go to the carbs and also the cooler.
The sort of weather we're having at the moment might initiate carb icing; cool but humid, so it wouldn't be a bad idea to have the warmers turned on. It also helps no end to blank off the air to the cooler with a wrap round affair of some sort. It will not overheat until the air temps get well on their way to the 20's.

I think it is definitely worth your while trying it again with the vacuum hole blanked and new spark plugs, before you wrench the carbs off. Also check the throttle and choke cables for free play, either could be holding the butterflies open if they are out of adjustment and upsetting the tickover.
It would be rather frustrating to find nothing wrong with the carbs after going to the trouble of removing and stripping them, not to mention the expense and wait for diaphragms ect… That's not to say that you don't have a carb problem, but I personally would try all the cheap and easy things first.