What is this duck- seen on a canal in Cheshire yesterday

We saw this duck on its own on the canal in Cheshire yesterday- - well away from the normal mallards- and was constantly diving under the water and swimming some length each time before re-surfacing. I thought initially it was a male tufted duck from the colouring but on closer inspection(as best as I could!) it had no had tuft and the beak was considerably longer than a tufted duck - any ideas please!