Somewhat later than intended, but here are some photos of the exhaust system chopped about, flanges and flexi sections added and then ceramic coated inside and out and the two into one section towards the back sprayed with VHT paint. First up, I present the new flanges:

The welding is of excellent quality (as always with these guys) and they've thoughtfully welded together the two very adjacent downpipes on both side of the flange to give extra strength.

The flanges themselves are made of 5mm steel plate. I'd originally asked for 10mm plate, but the angle that the downpipes entered and exited the flanges meant that thinner material had to be used. None of the pipes enter the flange 'square', which also accounts for a few small gaps between pipe and flange on the mating faces. All joints are gas-tight though.

On Eric's recommendation I fitted flexi sections on the twin centre section. When I took the two section to be ceramic coated I asked for the flexi sections to be left uncoated, but my instructions were misunderstood it appears. It's no big deal, but it just made welding the back section onto flexi section more difficult. You might notice a slightly different shade of black on the back siamesed section; that's the VHT paint I sprayed on when all the welding was complete.

And now a few of the two halves bolted together with a pair of rusty old M6 bolts.

All flange holes are M6; we reasoned that a number of smaller bolts were better than fewer larger bolts (if you see what I mean. Time will tell, I guess.

I'm not intending to use a gasket, just exhaust assembly paste; again this is something that may need revisiting in the light of experience, but I'm trying to keep it simple for now.
Cost overall were less than I expected. The local (Sheffield) engineering firm that fabricated the flanges only charged me £50; I was expecting at least twice that amount and maybe a fair bit more. I will definitely be using them again. Welding was £20 (again, a bargain) and the only expensive part was the ceramic coating (By Camcoat in Warrington). Including return postage, this came to £235.
Anyway, I'm back in France and rapidly running out of time. Hopefully before I return to the UK this weekend I can at least bolt the top section of the manifold onto the cylinder head. Never enough time...