Suspect Ignition Master Switch

My guess would be the headlamp flasher in the LH switch.

yes I have a sort of the same problem, when I turn on the ign the main headlight flash once don`t know what that is all about but it do that everytime.

but with you check out the light switch on left side 

Sounds like incorrect switch, there was a batch of these kicking about in the '80's and 90's. From what I remember you had to fit a diode across two of the terminals to get them work correctly.  

Another thing to think about along with the great advice you’ve already received…

The flasher button (push up the horn) is wired independently to the rest of the lighting circuit, and will operate in the ‘ignition only’ and ‘ignition and lights’ key positions.

It is fed by the same ‘hot’ white cable that feeds the horn.

If the rest of your lighting circuit is behaving as it should do, this is where I would start looking.

The spring could have become weak, or there could be some of the dreaded verdi-gris on the contacts.

In the ‘ignition and lights’ key position (key switch fully clockwise) the pilot/tail light/instrument lights should be on.

In addition, the right handlebar switch should turn on and off the headlamp.

With the headlamp on, the left handlebar switch should toggle between dipped beam and main beam.

In the ‘parking with lights’ key position (key switch full anti-clockwise) the pilot/tail light/instrument lights should be on.

The headlamp should not be on and the right handlebar switch should do nothing.

A couple of resources that may be useful:

Norton Commando Ignition Switch

MK3 Commando Console

Colour Wiring Diagram