Hi guys - I am back and so is my RMA old motherboard with new replacement Corsair TR3X6G1600C9 memory
Turns out it was a thermal problem with the memory on cold boot - once it warms up it was OK. The tech support guys confirmed this in their tests and it worked fine with their test RAM. I got them to test it out with the replacement RAM before sending it back. I have a new set of Corsair TR3X6G1600C9 - same model with different serial numbers (I had noted them done before sending back).
So far so good - it booted fine last night after reconnecting everything. There were some POST errors warning the hardware settings were different to the BIOS screens - I cleared the CMOS and loaded the optimised settings - this now seems fine. I have just cold booted this morning straight into Windows and then Memtest86+ without any issues!!
It feels too good to be true... I will wait a few more days before celebrating - it has been over 6 weeks now to get this far. Basically a dodgy chip on one of the DIMMs caused such a nightmare and hassle.
I will keep everyone posted. If anyone has similar issues I suggest RMAing the board and RAM back to the supplier along with a detailed note of how to replicated the prolem and your setup (as recommended by Dark Mantins previously).