Shatterproof is essentially the overflow for the In My Sights tank. I met up with troubleuk to have a look at the downstream section.
The tunnel around here is brick and stone making a a nice contrast to the newer section I know to be upstream.
Just before the outfall a waterfall drops down around 6ft tall, sadly there's no ladder down.
Moving upstream, the pipe changes to CMP before entering a pit. This appears to take the normal flow from further up, it goes through the wall into an adjacent chamber where it's pumped off somewhere.
To be continued...