Diving bird on River Kent today

Saw black diving bird on River Kent outside Kendal today. Only managed a poor picture as it spent much time diving and then flew up River. Feathers looked totally black and beak looked straight. Beak was black I think. Could not see any other colouring. Thought it must be some type of Diver but cannot find one that looks similar. Was by itself. When it dived it stayed under for a while @ 10-15 seconds. Neck sat at 90 degrees. Bigger than a mallard.