G'day Foz,
Don't know if it will help at all, but I have encountered the same problem with my 750 Mighty Demon.
First thing is to check for a genuine vacum leak, this is done easilly by spraying carby cleaner over all gaskets, hoses and seals on the carby and manifold while the engine is running. If the revs pick up while you are spraying, you have found your leak. Also check that your new master cylinder is not the source of the leak.
The biggest thing that effects the idle quality of the demon carb (assuming that your jetting and air bleeds are correct), is the position of the primary and secondary butterflys in relation to each other. If they are not set exactly the same you will get a rough idle and the car will stall easilly. Also if they are not set correctly, you can get a whistling noise that does sound like a vacum leak. So.... back the idle screws off on each of pair primary and secondary butterflys and re-adjust them so that each set of butterflys are open the same ammount at idle...... If this doesn't work, the best place to sort a carby out is on the dyno.........I can strongly reccomend barries dyno in Cannington.
Good luck on Monday