Thing is, I don't know what is normal for this bike, just comparing it to the GS500 which on a cold engine with the choke out starts almost instantly. The S40 on the other hand even when it is warm with no choke it turns over say 4 times, maybe more. It's not excessively long but  I wondered if this is normal and is caused by the auto decompression OR is it a symptom of being slightly lean?

This really can vary from bike to bike.

If you have the stock vacuum petcock, and your bike has been parked for a while....it may help to put it on PRI before you try to start the bike.

My bike start on the first or second rotation - I have heard others that crank over for a second or two.

Gary_in_NJ wrote on 08/10/16 at 06:29:53:

If you feel it's the decomp, loosen it a half turn till it starts immediately.
Then I'd say tighten a 1/4.

Takes a long time, be patient.

You may have a weak battery...

@Batman, I didn't think of that.
@Serowbot, I'll stick a multimeter on it too bad it's such a pain to do easily, I suspect the battery is fine but I cannot be sure.
@verslagen1, plugs good, only done 3,000kms air filter seems ok.

OK here's a recording of a typical start up. The bike has just come back fro a 20 minute journey maybe sat for 15 minutes but still quite warm. I am giving it no choke and a tiny bit of throttle, maybe 1/8th turn.

Notice after it starts it's idling at it's normal speed no problem but if I blip the throttle, the revs drop down, I give it a second larger blip and it slows down to the point of stopping!

https://youtu.be/fsNcouaBQ_0

OK thanks. What about the way that the engine speeds drops way down after a throttle blip, and an extra large blip causes the motor to cut out! you can hear that at the end of the recording. Is that normal? This is a warm engine.

Stock carb jets?

When you blip the throttle...you are putting the engine in a place it really doesn't like to be.  The open throttle results in the engine speeding up - but then you let the throttle slam shut......and this results in a high engine vacuum and very little fuel flow, as the slide is closed and the tapered needle has dropped into the jet and the only fuel flow is from the idle circuit - and the resulting lean mixture doesn't want to ignite very well......but once the engine speed slows down and the high vacuum condition goes away, then the fuel mixture goes back to normal and the engine returns to a normal idle.

One thing you can do is avoid blipping the throttle, as that really isn't anything that helps the engine to function (just a "Hey notice me" kind of a move that the biker dudes like to do as they arrive).  You can also make sure the idle fuel mixture is set properly and not  on the lean side....and you can raise the idle speed a bit.