More info on my mystery duck - he's got mainly white head and underside, mainly black wings, all markings are assymetrical. Dark eyes. More petite than a mallard, rounded head, yellow and grey rounded beak about half the length of the head, yellow legs, tail short, broad and dark without mallard curl. Imagine one of those black and white pigeons, but duck-shaped. There's nothing else even remotely like him on the canal, so he could be an escapee rather than related to the mallards.
Mmm, tend to agree with wildwood Kit. The only small duck that comes close to your description is the Smew. It's also known to interbreed with the Goldeneye, so that could reinforce the thought that it's another hybrid.
Thanks anyway, folks - looks like he's going to stay a mystery. However, he escapes becoming Sunday lunch by virtue of my being the planet's third worst cook. :)
Just a thought Kit - Do you have a wildlife park or an RSPB reserve near you? If you do it might be an idea to take a photo and ask them to identify it - then we'll all know!