The filters definitely worth a go. When you mentioned gunk earlier, was there a lot? Emptying the filter bowl may have got rid of a lot of it, giving you a good flow again, but it's built up again and is partially blocking the filter? This is a long shot, but perhaps google diesel bug (again, due to you mentioning gunk). I read about it when running biodiesel, and put an additive in to prevent it. I'm under the impression pump diesel can contain bio diesel these days, up to 5% without being declared.