The building through the gap in the distance is part of the hospital but has an interesting history that surfaced during research of this post. Will blog it soon.
Notice the street name below the reservoirs, Grand Junction Street, now Sussex gardens. Further down the page is a link to an 1817 map and you can see the canal company was here all alone giving them no opposition to name things as they wish.
The North reservoir is here in London street and has been built on. All three were sold 1842-1851 and this site was shown on ordnance survey maps of 1872 (click 1872) as being part of St Mary`s hospital. The map also shows this side street as being called Amber Mews and Francis Mews running down the other side of the building. Today they are Winsland street and W`mews.The part to the right of picture is to this day still part of the hospital. The GJWC built in 1855 a larger reservoir in nearby Kensington. Just 50 years later and the GJWC became Metropolitan Water Board.
Links for further reading HERE HERE
1817 map showing the basin, 2 of the reservoirs and the pumping station HERE