Bazza,
Is the Mikuni a secondhand unit that has been unused for some time? Over here in the UK, the modern unleaded petrol dries out to leave a sticky sort of lacquer coating on the inside of the carb, and after a winter layup, can cause problems when trying to get the fuel flowing again. I would check that fuel is actually getting through the jets - with the throttle open, put your hand over the intake and give the engine a couple of kicks. You should feel a strong suck on the intake - if there is no suction or even a build up of pressure, you have probably got the valve timing wrong or a tappet is too tight. If there is a healthy suck (at this stage, this is probably more important than having god compression) - check the inside of the carb to make sure that fuel has been drawn up. When checking for wetted plugs - check both of them - the inlet manifold on prewar Dougs can cause quite a bias on the carburation, so that one cylinder may look to be starved of fuel. Having got fuel through to the plugs, if the bike still wont start, then double check that the ignition timing is correct and that you have a good spark. Dont just rely on the timing marks - go back to basics - get the front cylinder on TDC with the valves on the back cylinder rocking - turn the flywheel backwards about 30 degrees (1/12th of a turn) and check the magneto points - they should just be opening. If the timing is out, remove the mag and turn it so that the points are just about to open with it firing on the front cylinder - then replace the mag. This should give you a good starting point (sorry about the pun) for final adjustment of the timing.
Good luck,
Eddie.