I quite take your point, Murdoch and yes, there's always that danger.
My reasoning for the approach I've used is that I now have a Sparks I can trust and if I do it right he doesn't have to run around sourcing bits and I get a good job done at a good price.
Like I said, I'm retired, I have the time and if an electrics job gets done this week, or next month, generally it doesn't matter. I have plenty to do renovating and building the rest of the house.
Without any shadow of doubt, I've found that shopping around - and having the time to do it - saves a LOT on materials. The Sparks can't afford to spend half an hour on the internet looking for the best deal but I can. He has to go to either the nearest supplier, or his regular supplier and accept what they offer. An instance of this was the sourcing of the RCBOs for the DB. I got most of them, all new, ranging in price from £18 to £24. The ones I didn't have, or that weren't in stock from my usual supplier, had to come from a local supplier (the exact same one the Sparks uses) at £45+ each. He ordered them, I collected them - and it was painful. From experience I've found that once a supplier has your sole attention, the prices tend to drift upwards. It certainly applies to Builder's Merchants as well.
Pete999: Your hopes are fulfilled and I'm afraid you are mistaken - and I'm not in trouble. Yes, I'm pretty confident I could do it safely and properly and I have in the past. I built dozens of phase converters for individual motors and connected up lots of machines but things have changed, especially the rules and the consequences of it going wrong.
I'm most certainly not doing the work this time, unless you call aiming to make a few holes in walls, screwing on a box, or two or running a length of conduit 'doing the work your good self'. Maybe it was the way I wrote it but the whole point is I'm not waiting to the last minute to source bits. I'm trying to get ahead of the drag curve.
Unfortunately I seem to be a lone voice trying a slightly different approach. I'm spending more time reading and answering reasons why I should not take the approach, rather than reading about what I should be looking for.
Will