Spoke to the mechanic this morning.
It turns out the person I spoke to on Friday afternoon was the owner, and not the mechanic working on my car (he had left for the day).
The mechanic took the Main Relay into a specialist over the weekend and here's the good news:
"He has confirmed there is dry and cracked solder on the Relay".

dougie_504 - I've rep'd you


Now to wait for an update from the mechanic later in the day to see if it's fixed...