Peugeot 406 HDi power loss?

I have a 2002 Peugeot 406 HDi GLX. Up until recently, it has been driving fine. It has always been punchy, responsive and quick... About a week ago, I have noticed a reduction in the performance; it accelerates, but not as well as it used to, infact it feels like I'm towing a trailer or a caravan ! It gets upto 80 - 90 mph, and sits there comfortably, but seems to take an age to do so. I've not noticed any reduction in fuel economy, lumpy running, black smoke ( the general things you associate with injector problems ) and the car appears to run fine, its just this 'diminished' performance that's puzzling me. Can anyone help ??