A final update in case in helps anyone.
I decided to pull each fuse one by one and then try starting each time, so see if any of the circuits was the cause of the fault.
The first fuse, by coincidence was labelled as 'alarm.
Pulled the fuse, tried the key and nothing (as you might expect)
Put the fuse back in and tried again - car started perfectly, no clicking of the starter, all good.
Tried starting on the key about 10 times and every time perfect.
took it for a spin and came back, again started perfectly.
All this was with the alarm wires removed from the ignition - so I then reconnected the alarm wires back onto the ignition, and amazingly, the alarm and immobiliser now working fine and car seems to have sorted itself out.
Took it to get fuel last night and was a bit worried it might not restart in the garage, but all fine so far.
Perhaps pulling the fuse reset the alarm somehow (although I have disconnected the battery quite a few times over the last couple of weeks.
So far i have put the dashboard back together and all good, so fingers crossed!
Thanks for all the help and suggestions along the way.

Pulling and replacing the fuse may well have cleaned up its contacts some allowing a better current flow, a new fuse and contact cleaner could help in the long run or at least you know where to look if it happens again.

I'll bet any money the connections at the back of the fuse had been pushed out a bit as well. I've had this problem plenty times

Hello.
So assuming the alarm/immobiliser was the issue (which I don't have), does anyone know of any repair kits for the NAD500210 model starter motor? The one referenced above seems to be for the newer TD5 starter(?)
I'm on my way to having three of the things tucked away in my garage... which doesn't seem right