ᐅ Wastewater drainage for patio roof – 80 mm to 75 mm

Last year, we installed a patio roof. Now I would like to drain the wastewater into our soakaway. There is a PVC pipe from the company DYKA inside one of the support posts. At the end of the post, an 87° elbow from DYKA is glued to the PVC pipe. The internal diameter is 80mm (3.15 inches). Unfortunately, I cannot find PVC pipes and elbows with an 80mm (3.15 inches) diameter. Apparently, this is a Dutch standard. Is there an adapter from 80mm (3.15 inches) to 75mm (2.95 inches) so that I can use an HT pipe and later a KG pipe? Or does anyone have a source for DYKA 80mm (3.15 inches) PVC pipes and elbows?
Thank you very much!

I hope you mean rainwater and don’t have a toilet on your terrace. In any case, I would never, ever direct wastewater into the cistern.

(SCNR).

There is online a: zinc reducer downpipe with a reduction of d= 80/76mm (3.1/3.0 inches) or a transition to HT 75mm (3.0 inches), I might give that a try.