You can't absolutely rely on the microfishe images that parts suppliers
use . They often combine different years models but don't always
differentiate as to which model had updated parts .
I have a combined RT1/ RT1B parts book from back then .
The decompressor valve was only fitted on the RT1 B .
The RT1 had an advance mechanism on the ignition rotor ,
and  the spark was retarded for startup .
Two different ways to skin the cat .

Looks like I have a B model then.

Also, I wonder if anyone can give me any starting tips re the decompression lever / valve. Found one on internet but in Japanese.

Comparing the engine and frame numbers (if different) with the VIN tables should confirm what model(s) you have. Both numbers will have been the same originally.

Apologies if you have done all of this already!

Are you going to use the original valve? If yes, have a look at the parts manual to see what you are missing and then have a look on fleabay.

Sorry if I misled. I have the original front decompression valve and lever etc. All I 'm missing is the rubber type cover. I just wondered if it is a simple as opening the valve before starting and then closing it once running.

The starting procedure is indeed that simple .
Good luck finding a rubber cover , they are a rare item .
I'm a bit surprised no aftermarket rubber merchant has
repopped one .

Now that would be good. I'm trying to find a source to see if they can be 3D printed.

The various manuals in the Tech Library explain the starting procedure and a lot of other good stuff! 

I seem to recall that Julie(?) from Sumo Rubber was looking for a donor part, with a view to using it as a template to get repops produced. No idea if the original gets destroyed in this process, which is why I haven't volunteered!

I also seem to recall that one of our US friends found a cheap item from a hardware store that could be easily adapted to fit.