Returning to plumbing:
Measured as accurately as I can along the pipe centreline from the very top of the tundish to the very edge of the T&P valve on the cylinder, is 960mm. I believe that's another fault, and the limit is 500mm.
However, I don't find any comment on bends or elbows affecting that length. I have two 90 bends, a 90 degree elbow and a 45 degree bend in that length. Does that matter?
With regards to the pipe downstream of the tundish, it is 22mm, I think I have just under 300mm straight pipe before the first bend, then I have about 5.2m nearly horizontal (but it does fall the whole way) then a 90 degree elbow and about 2.5m vertical before it terminates just above ground level in a narrow gap between my house and my neighbours (my neighbour has no access there, I have if I squeeze through a gap in the fence or climb out a window).
As I read the cylinder manual, this is another fault - 9 - 2 x 0.8 gives me 7.4m max length - I have about 8.0m. Actually, I'm not sure it's one elbow at the end - I've a suspicion it's three (I can't see how the pipe goes in where it goes in and comes out where it comes out, if you see what I mean) which would bring the limit down to 5.8m. I'm going to investigate that.
The discharge point probably does meet requirements - it's less than 100mm above a bed of gravel that's over rough old concrete from a former garage slab.
From primary circuit relief valve to the tundish is about 600mm, and from inlet group relief valve to the tundish is about 500mm.
Also, I commented earlier about no balanced cold downstairs, despite there being a mixer shower down there. The suggestion was that as long as my incoming mains pressure wasn't really high it would be OK not taking an actual balanced supply.
I've bought a gauge thing (pressure gauge on a little length of braided hose on an outside tap adapter) and it measures 3.4 bar (bearing in mind I doubt this is a highly calibrated gauge) without any water flowing. Given that the tap I measured at is about 3m below the inlet group, this means unbalanced cold pressure is going to be substantially identical to hot pressure.
That, though, assumes there's not significant seasonal variation of mains pressure. I assume there isn't (I can't see how there would be) but presumably summer mains pressures aren't higher than winter pressures?