Mechanical advice for a novice!

If you are completely lost, go along to your local NOC branch meetong and cadge some assistance.  Once pointed in the right direction, you'll need to get some spanners and a few good books.

Many of us first started to realise that motor vehicle maintenance was not the preserve of magicians simply by watching one of our neighbours working on an engine in the street outside his house.  That almost never happens today and young people are entirely alienated from their motor vehicles.

Meanwhile, put it on the stand. Prop somewhere to get the rear wheel off the ground. Put it in gear. No matter which  but try each in turn.  Remove the chaincase inspection cap. Try to turn the back wheel.  And see if the primary chain is trying to move.

Does it still kick start?

if so, the primary is/maybe ok. Then suspect the g/box and final drive

where are you in the country?  

You say that you have lost all gears but is this the case? Does the bike still change gear? Usually when you lose all drive suddenly it is because the chain has come off. When following David's advice,  if the wheel turns freely and the chain is still on then suspect the primary side. 

If you post your location then we may be able to recommend someone to have a look.

 I think the most likely culprits are primary chain , the engine woodruff key, or the clutch nut . Also the easiest area to get into after removing footrest and rear brake pedal. 

Its been at least 50 years since my clutch came loose but I remember the scraping noise at it rubbed the case. There could be several reasons for this . The clutch was not tightened up enough,  the spring washer under the nut was ommited or not a close fit . Its less likely that the clutch center( spider) is failing and moving up the shaft (allowing the nut to loosen) but it did it to me.I now use a little locktite solution on the nut and use less tightening torque than specified. The primary  chain needs some slack to avoid straining everything , and its often wrong due to its tendency to get pulled tight if the box moves back.  Lots to learn ,but we all started somewhere. Please let us know what you find .

It might be worth getting along to Oxford branch meeting and seeing if someone is able to help.

They meet at 7:30pm on the the second Wednesday of every month here:

‘The Red Lion’ The Green, Cassington, OX29 4DN

You are about an hour away from me, if the weather is good enough I could possibly pop over some time over the weekend. I will send you an email once I see how the weather is.

Regards

Tony

My guess is broken primary chain. You should be able to see it through the inspection cap.

dan

You have hit upon one of the Dominator's Achilles heal's. After years of over tightening the cover will be distorted. Plenty of sealant together with a good or new seal will probably be necessary.

I would not recommend using the bike on a regular basis without the recommended quatity of oil in the oil Bath.

... you probably know this but after adjusting the chains make sure you finish up by pushing the gearbox forward ie in the direction to slacken the primary. This is because the secondary chain exerts more pull on the box than the primary so tends to pull it backwards. Doing as suggested takes up the slack so this is less pronounced.

It does sound as though the bike has been rather neglected by the way.

The chain is most likely to have had a hard time already from the sound of it . A new chain would be a  prudent investment.

I run open primary on my bike. Just replaced the primary with a 530,  Lube when I get back on tick over.  The chain oil sets when cold and stays in place far better..  With the oil bath do the same with a spray lube through the inspection hole. 
J

As a lad, I regularly threw chains on derailleur-equipped bicycles (on the unpowered front, I've seen the light and only run Sturmey Archer epicyclics these days)...I can't imagine though how a primary chain no matter how slack could come off both chainwheels. Did it catch on something, or were / are primary sprocket or clutch running out of line ? I'd worry a bit about this. Random transmission-throwing is generally the realm of the belt drive enthusiast.