It's been 40 years since the reservoir was that low - I have some shots of it from then.
In the sixties the church steeple was a regular sight sticking up out of the water - until it was deemed unsafe and 617 squadron paid a return visit to their former training ground and bombed it!
This surprised me as outside war zones, the RAF takes a dim view of chucking bombs anywhere except on ranges approved for the purpose so I checked. I am not saying your recollection is incorrect but according to Wikipedia, the clock tower of the church had been left standing and the upper part of it was visible above the water level until 1947, when it was seen as a hazard and demolished with explosives on 15 December. Citation- The Story of the Lost Villages of Derwent and Ashopton- Harry Gill.
PS. In the 60s 617 were part of the V Force flying Vulcan Bombers equipped to launch Blue Steel nuclear missiles and later standard nuclear bombs. Had they had a go at Ladybower Reservoir I think it may be bit deeper and probably off limits for a fair while!
I stand to be corrected as I have only used Internet sources and have no other source material.