Here's what you have all been waiting for. After a long wait, I did this job today. Some pictures.
Before (well nearly I had just started untangling it then remembered to take a photo)
There were a lot of redundant circuits, some unlabelled, and some with completely the wrong size mcb (16A lighting, 40A immersion heater)
By the time I had worked out what was really needed I could populate the CU with the rcbo's. I over ordered on a sale or return basis so I had more rcbo's than I needed so I stood a fair chance of having enough to get it right (the joys of doing a job over 100 miles away from the nearest wholesaler)
Note the gaps between the big rcbo's. the right hand N bar is only being used for the 50A rcbo and one of the 6A ones. All the rest are on the left hand neutral bar.
And here it is at the end of the day.
This is using rear cable entry and SBS Dave's rear stand off frame. Most cables then exiting at the left but a few come down out of the ceiling.
And that 50A load, the electric boiler. I measured it at 48.3A. It appears to have a soft start in that it takes about 3 minutes two get to full load, I think it's switching in heater elements in banks. I ran the heating for half an hour and checked all joints and everything was solid and not the slightest hint of anything even getting slightly warm.
Just as an aside, this job was about 75 miles from my home, over on the north west coast. The journey took 1 1/2 hours each way and at no point was I even following another car, let alone held up in traffic. It was a glorious day up here in the northern Highlands, and I wouldn't wish to be anywhere else.