Ok - so I drilled out the two bottom rivets that hold what looks like an access plate.
Sadly having done this it does indeed reveal the internals but it is quite clear this part is not designed for refurbishment, it is a sealed unit.
What happened to mine is, I suspect, due to aggressive turning of the key (I've owned this Spitfire for over 10 years so take full responsibility), it had sheared off a semi circle of the barrel mechanism that was inhibiting the turn of the key.
I was surprised not to find an easy way to just remove the barrel for a key swap (maybe after a broken key) this would not have helped my situation as it was the barrel casing that had sheared, but I wonder if a more experienced lock smith/expert could remove just the barrel.
Despite being nearly 40 years old, the internals were remarkably clean and well greased, despite having made the mistake (as pointed out in the feedback) of squirting in some wd40.
I think - if it happened again, and due to the lack of parts and the cost if available, I would be tempted to fit a starter button to the dash.
After all - who doesn't like a big shinny button that says start !!!!!!!!!!