I was involved with the one in Blackpool, they are a pair of deep shafts, 40m deep and 36m diameter. Working volume was 40,000m3. https://www.yumpu.com/en/document/read/35485775/storage-tanks-soletanche-bachyYes, I seem to recall something of massive projects in Blackpool, I think there may also be some in Lancaster and the lakes area. Not disputing what you typed at all, but the report I read on it claimed diocletian to be bigger, but I agree with you, I think in my sights is much bigger overall.
There are another 2 at the treatment works at Fleetwood. This is going back 25 years, I think they were 60,000m3. Part of the Flyde Coast project.
I also worked at Bolton MBC - Drainage, we were famous for the big tanks. I remember this one being built and the Engineers involved.