Realizing you're in the U.K., while my experience is here in the U.S., I suspect the cause is similar to problems here... If you look closely at the lower leaves, or any leaves on the corn plant you'll see they are veined... that is the ribs you can see and feel running length wise are the sites usually attacked first by diseases as well as entymologically based predators. If the leaves don't display any signs of discoloration (brown, grey or red-rust) around small holes in between the ribs, the cause is unlikely fungal or viral in origin. (Here in the U.S., the most common would be a form of Mosaic , a virus) If there are discolorations around small holes that later lead to "shredding" then an application of anti-fungal spray is in order.
In the absence of discoloration, there should still be small holes that lead to the shredding you see. These are caused by any number of preadtors... opportunisitic little critters, that can also be controlled by judicfious application of various sprays.
There is the off chance that you've planted the crop in an area of your garden that was sprayed last season with pre-emergence weed killer. There's also the off chance that you simply have a need for additional nitrogen or phosphorous fertilizer.
I would expect the cause to be bugs. Take a close look at your remaining plant's lower leaves, especially the undersides and see if some of the usual small specks you might have thought were dirt, move. If they move... treat them.
Best of luck...