Here is a follow up on the issue mentioned above.
I followed the advice to bypass the starter relay and replace it with my own 30A relay. I used a fused, 30A automotive relay (4 pin - 85, 86, 87, 30) and positioned it in the engine bay in the far right hand corner, when viewed from the front of the car. I thought this way I would be keeping it away from the heat as best as possible, and also close to the hole in the firewall that I used to pass the wire heading to the ignition block.
Initially I thought I would use my own normally open switch to activate the low current side of the relay so I could avoid tinkering with the wires on the starter block. In the end I thought the added annoyance of needing to use a switch separate to the keys to start the car out weighed the novelty. With the supply wire disconnected from the starter motor (see attached pics) I cycled the key through the three positions (ON, ACC, STR) and noted the voltages on the wires at the back. This trial and error process allowed me to find a wire that went to 12V with the key in STR, was otherwise 0V, and was not being used in the loom. I am not sure why it would be like this, perhaps an additional option not utilised on my car? Anyway, I used this as the supply (low current) positive for the 30A relay, and wired the other side to earth. Then wired the high current side from the battery through the relay to the supply on the starter motor. Worked first time and haven’t experienced any issues since!
Thanks again for the advice. I have added some photos in case anyone else experiences this issue and needs a head start. Note: I found the easiest place to pass a wire through the fire wall was on the left hand side of the car, directly behind the glove box. There was a rubber grommet there which could easily be removed.