Yes, both Jim and Pentaxuser are on the money here. Perceptol is Ilford's functional equivalent of Kodak's Microdol-X, and I've used enough Microdol-X in my time to verify what they've written. At full strength, speed loss can be as much as a full stop and gain that "fine grain" effect. However, once you dilute to 1+3, the situation is reversed. The fine grain effect is lost, but full emulsion speed is achievable.
Anyway, 320 is only 1/3 stop slower than 400, so I wouldn't worry too much about it. Extending development a minute or so over the recommended time might boost the shadows a bit, but only if there is some detail there to begin with.