Well, I've got it working - sort of.
To start with, the machine refused to boot when I started it. All the fans would come on but it would just hang there and not post - it just did nothing. I tried all sorts of things; reset cmos, took the battery out etc several times. Then I had a look around and it looked like the backplate was contacting a couple of connectors on the back of the MB so I took it off and re-fitted the block and, hey presto, life.
Of course, I'd reset the bios so I had to start from scratch. This board is a bugger to OC and it took me ages to get anywhere with it. I partially blame my choice of drink for the difficulties I had. Instead of the traditional whisky, I fuelled myself with 3/4 of a bottle of Booth's gin
At the moment, she's Folding away at 4.2GHz but that's with the fans on full tilt (those Triebwerks are quite loud at full speed) and the temps are not good - 86-80-81-75. Vcore is only just over 1.4V and I'm not happy that it's 100% stable at that. The fact that the overall temperature is so high with a relatively low Vcore and there is an 11 degree difference between two of the cores would suggest the block isn't seated properly. I think I'll take the block off, reapply TIM and reseat and see if that makes any difference. I'm pretty sure the flow is OK because, when I put the dye in, it went all the way around the loop in a matter of seconds - wish I'd bought a flow indicator now so I could be sure.
I'm not sure about this CPU, it was running hot before as well. When it was at 3.8GHz under the Noctua, it was over 80 degrees at 1.376V. The new rig, under the same Noctua, runs at 4.2GHz with a Vcore of 1.4V and rarely hits 80 degrees. I have a sneaky suspicion this may just be a hot chip. If reseating it doesn't make any difference, I may swap the two CPUs and see if that makes a difference.
Might be the board that's at fault. In which case I might end up swapping the Gigabyte board into the gaming rig instead of the Bloodrage. I want to have the fastest, most stable setup as my gaming rig. Swapping the boards would ruin my red/black colour scheme but I value performance over aesthetics. I'm not going to mess around with it just now - I have a hangover (bloody gin)

The current WU is due to complete in about 4 hours so I'll let it finish that and then faff about with it.