Quick update for you then, AS IT'S NOW FIXED WOO HOOOOOO
The relay was spot on, the battery was spot on, the charging system was spot on. I had the battery tested etc and it was all good.
It turned out that once the starter motor gets hot, the internal resistance increases, and the brushes, being on an 8 year old starter, have got a little tired these days, and need to use a little more power to get the motor working.
I actually had a 12v6AH battery in the bike, unknown to me, 3AH less than the standard fitment, so despite working fine, it just didn't have the ooomf to start the bike. I bought the right sized battery and it's spot on!
So it was the battery, although the battery was working fine, which i pretty much had worked out, which was the thing that was so hard to understand!
Thanks for everyone's input and help on this. 'Tis the very reason i use forums when everyone helps out and does their bit to get things sorted!