Pete,
All three of my carbs have been completely stripped down and cleaned thoroughly, its was shocking to see the amount of crud that had built up in the carbs. That crud might be blocking the needle valves from your floats. But as yours are dripping fuel even with the engine off I'm thinking its a fuel boiling issue.
Chris Sale sent me a great piece he'd written on the issues he had with ethanol and fuel boiling in the carbs which I have attached.
Basically the increasing amount of ethanol in fuel these days results in the fuel boiling in the float bowls on a hot engine, even when it is switched off. When it is running there is enough fuel flow and air flow to keep the carbs cool and the fuel from boiling in the bowls. But as soon as you switch the engine off the residual heat boils the fuel in the bowls causing it to leak out of the carbs.
With mine, having left it overnight, it burst into life first time. I realised that I usually lift the bonnet as soon as I stop for any time longer than it takes to put petrol in, this must usually let the heat out stopping the heat from building up.
Hope this helps. If not get back to me.
Regards,
John