D
dextrous
The phrase "optimum gradient" springs to mind.
From the op's description, the longest length will be about 2500mm with a drop of 30mm (40mm pipe in 70mm space) which is 1 in 83 which is just within (well nearly ) the regs.
From the op's description, the longest length will be about 2500mm with a drop of 30mm (40mm pipe in 70mm space) which is 1 in 83 which is just within (well nearly ) the regs.