Annoyingly it was indeed the CPU. After returning the CPU and MB to the shop they were able to isolate the fault. At least the claim was painless.
Even more annoying, my NAS which is an old Dell workstation, died over the weekend. No post at power up, opened the box and saw a couple of leaky caps. <shrug> Its been running 24/7 for 5 years, and I bought it from Grays so its probably 8+ years old.
More annoying still, of the (desktop grade) high end components I purchased to build a replacement system, the MB is DOA.