The southern water bunker is much further away, totally different place.
I happen to Live very close (like 300 metres) away from the area you're referring to, but there is no underground reservoir in that area as far as I'm aware.. If you think about it why would there be, it's totally the wrong area for a reservoir as you're in the bottom of a valley. However, there are fairly extensive drains in the area that thus far I've been unable to get into despite having a good walk around looking for access (I know they're there I have the plans) and it's possible that this underground "jetty" might be a chamber or something relating to this - maybe I'll go have a more thorough look around the area. Southern Water still have a pumping station at the bottom of North Dane Way, but their old HQ buildings have been demolished, you're right.
There are two reservoirs located at the top of the hill just off ash tree lane, these are disused now and due to be demolished for housing, but thus far no work has started.