So finally got around to working out the final levels and the final plumbing design. Everything has to be aligned to the water level - I use a laser level on a tripod for this.
The garden almost just at the bottom of the picture is 630mm below the intended water line. The hole floor is 2090mm below the water line, the garage floor is 680mm below it. The garage slab is 150mm thick and the wall under the door goes down at least 680mm. The waste pipe being 1200mm below the water line.
The hold sounds deep but 8" or 35cm of that is concrete base with the bottom drain in there.
So you end up with all this referenced off the water line, and then the bio chamber needs to sit with it's base 830mm below that water line - or 150mm below the garage floor. Point 1 - a sump is needed. Next up is the piping - the return apipe is the deepest pipe that will sit above the existing waste pipe level, so doing the calculations - I have 370mm for the bio support slab and the routing of the 110mm return pipes.
Then.. take the pipe routing - the output from the bio is 130mm from the base, so far too short a distance to have two 300mm radius 90 bends to return plus a 110mm ball valve (big mofos).. so a compromise of a T junction for the bio output drops down to a 90 that then routes back - the ball valve is then installed on the up pipe just before it returns to the pond. Not much space there so I will need to check.
So next is to order the bits, then I can crack on with it as today is the last day of quarantine
TLDR - water levels for gravity feed filter are a biatch, plumbing doubly so.