Starting issue

Your very first statement of this thread " Ever since I had this bike, it's been hard to start when hot" should have triggered the question what changes when the bike gets hot? By adding heat, something caused a no start issue. So what gets hot from the engine heat? Answering that question should have lead you to look at the starter motor first.
In my lifetime as a mechanic, more so as an auto mechanic, I've seen more starter motors get replaced for this reason and as an auto mechanic all we knew was that if a starter motor was acting up, just replace it. Now further down the road of life and having  a millwright background, I've learned to investigate issues and understand failures and ultimately fix the problem rather than swapping parts.
 I wish we had a good "rebuild your starter thread" to follow. A lot of starter failures are due to engine starting issues and people trying to start the engine by repeatedly holding the starter on and not letting the starter to cool. Bad, Bad, Bad. Windings overheat and desolder themselves leading to total destruction, or the commutator rings get wiped causing bridging of adjacent windings.
One thing that happens often is a non electrical guy tries to rebuild a starter and paints all the parts to make them look pretty, assembles the parts and the starter won't turn over. You really need to understand the ground feed from the ground brush. Most starters have a brush plate and the ground brush is riveted to the brush plate. The brush plate is sandwiched to the field, which is sandwiched to the starter housing which is bolted to the engine which is attached to the ground wire to the battery. I've seen brush plates painted a fields painted causing a failed path to ground. If you decide to rebuild the starter motor, take the time to understand the ground path through the starter.

Thanks Kawboy. My issue seems to be that the solenoid doesn't activate when in situ.   I think the starter always turned over when the solenoid did activate. I disconnected the large cables to the starter motor in order to hear a solenoid click...which isn't happening.   Somehow when I hit the starter button the solenoid just isn't working. Yet when I give power to the solenoid from an external source it works every time.

I didn't repaint the starter as I thought it's hidden and no one ever sees it. I did try and make sure all earth connections were paint free and clean too. But as I say I think my issue looks to be further up stream.

"I rebuilt the handle bar switches " 
Did you actually take this switch apart?    Simply trying to spray contact cleaner into the switch is not enough.  I removed the two little brass contactors (only one actually makes the internal connection) and the "live one" was dirty with some minor pitting.  Physically cleaning the contacts solved my "intermittent" issue. 

Bypass that  P.I.T.A. switch on the clutch lever!  This could also be the problem; or contributing to it.

I did take the switches completely apart and cleaned the contacts. I used a little bit of dielectric grease in some areas after cleaning, more on the left switches than the right.  I hope that hasn't got on to the contact area..I tried to be careful,  I'll have another look just in case.

I've never liked that safety switch on the clutch lever. I'll be happy to loose that.

Well I got rid of the clutch safety switch. Maybe It was my imagination, but I the solenoid seemed to click more often when pressing the starter button.  2 clicks out of every 10 presses...ish.

I took the starter button apart again...while it was still on the bike, that wasn't as easy as on the bench. . I just didn't want to pull the throttle cables off again. 
The contacts were a bit black, which is very strange as I had rebuilt this probably 1k or 1.5k miles ago   .The black wire looked a tiny bit green where it meets the blob of solder on the starter button. Maybe the old wires are creating more resistance making the contacts arc more? Maybe the starter motor having issues when hot has me pressing the button  more? I don't know. I cleaned the contacts again and refitted it. The solenoid clicks every time now. So I re connected the solenoid to the starter and it turns over just great!, success! .

That was a cold start, I have a feeling it may still have trouble when it's hot, but at least that's progress. I'll test it out and see what happens after a good ride.

I tried to take some photos of the switch, before and after cleaning, plus the slightly green wire but only some of the pics came out usable.

Well.....there you go !    Sometimes you just have to reach out and "contact" someone
You're observation of some oxidisation on the soldered wire is noteworthy.  In my experience this can sometimes indicate a "cold solder" joint.  I had a similar situation with the high-beam switch.
When I went to move that wire for better access for the soldering iron - the entire pellet of solder popped off the post, cleanly.  A soldered connection such as these should be nicely "domed" and shiny. If it's dull - a potential problem.