Thanks people, much appreciated. Put a good battery in it today, switched on the ignition and got a neutral light and an oil light - Woohoo! Indicators work but nothing else. Took the points cover off and with the ignition on opened the points - Spark! Woohoo! Filled oil-tank. Found some gas in the shed and rigged up a remote gas tank - didn't want to pour a gallon into the rusty wreck to find it leaks like a sieve.
Choke on...choke cable seized closed; pry up choke lever on carb bank, switched her on and hit the button. Engine turning over but not firing. Checked for a spark at the plugs; all good but plugs dry; no gas getting through ?
I took the carbs off to clean out the expected gunge only to find them pristine, clean as a whistle. Had much fun getting the carb bank off, struggling against the airbox. All done eventually.
Then I remembered that the gas in the shed came out of the last bike I bought and was about 15 years old, I was keeping it as parts cleaner. Jumped on my MZ and down to the gas station with a petrol can stapped to the seat.
Ok, try again, fresh fuel in remote tank....sod it, I'll fix the choke cable. Managed to get it free but broke of the plastic plull knob. Tig welded a 5mm diameter metal bar so the cable end now looks like a 'T'. Much better. Choke on, Gas on, Ignition on, hit the button..krob-krob-krob-krob-KROBLAAARRRUMMMM. Running! Turned around to look at the expected palls of oil smoke coming from the exhaust...nothing...no smoke..I ease off the choke a little and the bike settles down to a steady idle. Niiiice! Hardly any engine noise at all; engine sounds like it's done about 15 -20K miles, a little dull clatter from the cam chain at idle but nothing to worry about - may well quieten down after I've adjusted it and balanced the carbs.
I decided to film the bike idling with my cellphone, not great definition but better than nothing. did that then had a close look at the gas tank - amazingly it's practically cherry inside. Tentatively put in a couple of pints of gas and swished it around. No apparent leaks so I poured a gallon in. No leaks at all!
Freeded off the sticky rear brake, & adjusted the clutch and took her out for a quick ride up the street - no front brake though so took it easy. Gears selecting very smoothly, clutch working normally.
Started to clear up. Bent down to pick up pan full of old gas that I was using as cleaner and my 'phone fell out of my top pocket and landed right in it. Glug-glug-glug as it sank to the bottom. It doesn't seem to want to work now so I'll have to post a vid another time.