Trenches have been dug for the plumbing. All of the return piping will be 90mm stormwater pvc pipe. I will have two returns to the sump. This diameter should be more than enough to cope with simultaneous draining. I have decided not to use the olive barrels as extra sumps, as I am pretty sure that the sump and the yabby tanks will be more than enough. There is potentially room for another growbed, and I will add an extra sump if I add another bed.
Should get the pluming done this week ready for the gravel by the weekend.