So, I've diagnosed my issue with regards my starter motor.
I originally removed the starter and took it to another local testing and repair shop.
They also couldn't really find an issue with it.
So I tried installing it with all the connections cleaned up, but still had the same issues.
I then removed the starter and left it loose but connected and tried to start it.
Boom it fired!
(it earthed itself to the gearbox)

So, I put it down to and earthing issue.
I removed the battery negative cable
(located behind the passenger seat) and installed a jumper cable to the same grounding point next to the seat.
Click click click!
I then attached it to the wheel hub and boom it started!
Tired the gearbox, boom started again.
So I feel like I've got an electrical earthing issue somewhere in the engine bay.
Either that or I've melted some of the wiring but I can't see any damage...
I did clear up one earthing points, however it made no difference.
So I order some four gauge black negative oxygen free wiring from eBay and I will install that in a couple of days when it turns up to give me a significantly stronger earth.
On a slightly serious note, I also could smell petrol in the engine bay and after an inspection, I noticed that the pipe to my filter had also split and was leaking fluid everywhere.
So this is another problem thankfully I found and I'll order some replacement fuel pipe in the next day or two and replace all of the pipes in the engine bay over the weekend as they're all looking a little worn and tired!
Plus... the last thing I need is a fire in the engine bay after all this work!