Osma gulley is a different design, but that type you have is absolutely fine, (I have 2 here I installed myself and never been an issue).
My suggestion was to sit the gulley with the grid at finished floor level, then extend the outlet pipework as required to meet the outgoing drain. So immediately into the outlet spigot from the gulley, fit a bend, angle to suit), then run from there with length of pipe, to meet either another bend, or rotate the junction to point the socket in the required direction to accept the incoming pipe from the gulley.
Its a hell of a lot easier to clean the gulley out/unblock it, if the pot is near the surface, rather than being 2-3ft down, and the height made up with risers.
My suggestion was to sit the gulley with the grid at finished floor level, then extend the outlet pipework as required to meet the outgoing drain. So immediately into the outlet spigot from the gulley, fit a bend, angle to suit), then run from there with length of pipe, to meet either another bend, or rotate the junction to point the socket in the required direction to accept the incoming pipe from the gulley.
Its a hell of a lot easier to clean the gulley out/unblock it, if the pot is near the surface, rather than being 2-3ft down, and the height made up with risers.
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