It turns out that the corn is not gone bad: it is actually not ripe yet.
I re-opened the cob leaves and the tiny pale yellow grains I noticed last time seem to be turning greyish. From that I derive there is a problem with the method of checking the tassels: it does not seem to be very reliable as the said tassels are probably very tasteful to all sorts of creatures from slugs to woodlice (I find one at the top of the cob) so they might go brown for other reasons than ripeness. I am feeling a bit better about it: I have obviously planted the corn too late... hopefully if we get warm weather this month it will still ripen.
Tonight I did not have much time and the drizzle was really sad and discouraging, but I managed to quickly gather vegetables for the whole of the week. You should smell it, it is as fantastic as it looks!
By the way, spinach seems to be coming out, but I am afraid that slugs might have been too much for both corn salad and lettuce...
Now back to the kitchen to cook dinner: tonight I am on a very a tight schedule!