Thanks for all the replies so far, very helpful indeed. To clarify a few things:
I don't think he wants the responsibility of lining up all the trades, taking on all the responsibility etc. He just wants to get on with things and would be happy to direct traffic on site and act as a kind of foreman.
We've found non VAT registered trades in the past, for example a brilliant gas engineer who I'd look to get hire again. We're on the outskirts of North London, just inside the M25, so hoping there are more of them out our way.
We'd assumed that to PM the job we'd need to be much more involved, i.e. I'd need to quit my job or seriously cut down hours. From the sounds of your advice and of others, that may not be the case. The flat that we're renting is 3 minutes walk away, so I can drop in on the way to/from work if required, and there may be scope to work from home, but this would not be on a regular basis.
As a frustrated white collar worker who's always dreamed of building a house, this is highly appealing. My job isn't particularly interesting and I think i could get another one pretty quickly after the build is completed (bit risky though). Saying that, the limit of my skills would be as an unskilled labourer and tea/coffee maker so other than as PM, my value add is limited.
Yes, we've looked in the area, and what we'll have when it's finished is better than similar priced properties and will be really unique. We're not 100% sure the value of the house would increase by £250K, but it wouldn't be miles above or below that mark and we're planning to live there for a long time.
There seems to be a consistent opinion, just from the responses so far, that keeping the architects will be a waste. We'll be having a meeting with them soon to discuss how they would support us through self-build, so if anyone can advise us on what we should be pushing for, or expecting from them, please shout. We've enjoyed working with them so far, and they've come up with a great scheme and lots of great design ideas, so not ready to kick them off yet, but want to get value from them at the same time.
Cheers