Depending on how much trouble you want to go to, Ilford Perceptol (like Kodak Microdol-X) reduces the speed somewhat. Depending on your metering technique and the subject matter, you might not have overexposed as much as you think. I would reduce the dev time a bit. Maybe 40% less is too much of a reduction. I'm guessing here.
If it was a highly contrasty scene, you might have exposed just about right.
I normally rate 400 speed films at 200 and use a generous exposure as well.