All was well with the install (minor BIOS tweaks needed to get the memory running at spec) after I set it up with just 1 of the RAM sticks ("A") in slot 1.
After running Prime on and off, to bed in the CPU, and a run of Memtest I put the 2nd stick ("B") in slot 3.
It booted up fine but then took errors in Prime and it stopped running on one of the cores.
Took "B" out and it ran fine again.
Bad memory for sure...?
I put "B" in slot 1, by itself, and Memtest showed it all clear!
So, worrying it's now a bad slot on the motherboard I did the following tests;
1) RAM "A" - Slot 1 - Passed
2) RAM "A" - Slot 2 - Passed
3) RAM "A" - Slot 3 - Passed
4) RAM "A" - Slot 4 - Passed