I think there is a bit of confusion (me included) as to what the Frankley reservoir was . The Frankley reservoir was the semicircular reservoir next to the waterworks seen on the 1937 map in post 172. North of it is the south part of an irregular reservoir, which is not shown on the 1913 map. this is the Bartley reservoir, and was completed sometime in the 1930s. The cottage referred to would have been under the Bartley reservoir .That’s interesting the data of the picture on Birmingham Archives and Collections says photographed in 1930, but the writing says the cottage was demolished for construction of the reservoir which was built in 1904.
Perhaps there were later modifications to the reservoir ?

I see. But surely swanky Sutton uses bottled mineral water?!! Sorry, silly joke!Not strictly on topic but this is a fact that I find amusing. Our postal address is Sutton Coldfield and halfway down the road used to be Erdington. After a re-jig the whole of the cul de sac is now in Sutton. It's only a very small road with 29 houses and our end is South Staffs Water so 'hard' and the other end is the nice soft stuff you get in Birmingham. When there's a problem both water companies turn up!