I have an intermittent diesel leak from around the back of the injector pump somewhere. The pump itself is dry on the outside, all around the injectors, glow plugs, the high-pressure pipes are all clean and dry; but underneath and behind that there is evidence of a diesel leak. The back of the car also gets covered in diesel when it is leaking, including the rear window, which is how I know when it's happening.

It only leaks on warm days when I am driving at about 60 - 70 mph, or if I am accelerating hard up a hill. The rest of the time I cannot make it leak even when I try (or at the garage when they try). We steam-cleaned the engine, then revved the nuts off it with the bonnet up, looking for the source of the leak. Then we put it on a ramp, more revving, but still no leakage.

I am not an expert on diesels, my mechanical experience is limited to motorbikes really, so I am not really clued up as to where I should be looking for the problem. There is a silver disc that is part of a supply or return hose (I think), which seems to be where the greatest amount of diesel is accumulating, but there is no obvious damage to the hose or the thing itself.

Has anyone heard of this before? Any clues or pointers? What is the silver disc thing for - it's really starting to bug me that I don't know what it is? Thanks in anticipation of your answers. At £1.45 a litre I can't afford to be leaking diesel like that.