Does anyone remember an old British movie with a flood?

When I was a kid (early '70’s), my dad took me to see a British movie about two kids in a town that gets flooded. Does anyone know what that might have been?

I remember one scene. The little girl starts crying about her mother being dead. Her brother says “we don’t know she’s dead until we see her floating belly-up.” Then, as if on cue, mum goes floating by belly-up.

I would love to know what the hell that movie was!