I had it this year too but it came quite early in the season so I lost about a third (if not more) of my crop.
Kind of amazing and impressive but you need to get rid of all of it, burn - no composting it as it'll spread the black spores. And now because they'll be more...and it'll come back next year. Apparently a heavy liming (30z to the sq yard) can affect it but there is no cure.
I've read it's best to avoid planting sweetcorn in the same spot for 3/ 4 years. Luckily I've space so I'm planting it at the other end of the field next year.
I was so disappointed as I had a good year last year, but just hadn't planted enough. I guess next year I'll plant even more in anticpation of possible loses. I didn't realise it was common it the uk as it's a hot weather thing so perhaps you'll not suffer from it again?
I wasn't brave enough to try eating it but this is a quote from an answer I had about it on another site...(thanks welsummerman!)
"thought this may be of interest as well copied from the american usda site
Corn Smut is a fungus that grows on corn kernels. It causes them to swell into grey, sometimes black, galls up to a few inches (5 to 7.5 cm) wide.
Sweetcorn is more prone to Corn Smut than are cattle corns such as Dent Corn. The Corn Smut has to be harvested while the galls are still moist inside and still whitish-grey or grey, before they become black. The fungus inherits a lot of the sweet taste of the corn that it is feeding on.
When cooked, the fungus turns black and has an earthy flavour. It has been a prized ingredient in Mexican cooking for centuries, and commands premium prices there.
Corn Smut is certainly not pretty to look at, and is heart-breaking to most North American farmers growing corn. The US Department of Agriculture (USDA) tried to eradicate it in America.
Around the same time, Mexican restaurants in America, began serving it, calling it "corn truffle". They had to buy it from Mexico, paying the premium prices. At which point, a collective gasp when up from American farmers: "How much are people getting for this bad corn???" Consequently, the USDA is now trying to figure out ways to actually cultivate and encourage Corn Smut.
In trying to fathom the thought of eating Corn Smut, it might help to remember that truffles are funguses that grow in dirt.
Corn Smut can be bought canned or frozen, or fresh during corn harvest season."