After traipsing through the undergrowth with Eotech we arrived at the spot. Blue disposable gloves and a big cheap torch lay strewn around the lid, evidently somebody else had recently been down. The coast was clear however and we descended into a penstock control/access room.
From there we had the choice of 2 ladders, each leading to identical tanks that ran parallel to each other for around 120m. Each was 3.5 x 6m which gave a combined capacity of around 5000m³.
We entered at the downstream end where solids tend to accumulate as the tanks drain after filling up, the walk to the other end was completely uneventful bar a frog hoping about.
The incoming sewer was around a meter in each tank, unfortunately there was no direct access to the overflow chamber, only another landing the same as the one we entered through. We called it a day at this point, however just upstream the overflow chamber serves a 1.6m sewer which can spill into a box culvert down to the River Bollin. JD has a photo on his page here.