Do NOT use diesel, Glyn. It will probably not kill the knotweed, it will contaminate the soil for years to come and you are liable to be prosecuted for causing pollution!!!!!
Japanese knotweed is a problem and needs to be tackled properly. You could try to dig it out, difficult with a spade, then bag it up and arrange with your local council which special waste site you need to take it to. Glyphosphate weedkiller such as Roundup is a possibility but this will need several applications over the next few years to get rid of the knotweed.

Very difficult to get rid of and a glyphosphate weedkiller is prob the best as Bulbaholic says- NOT DIESEL.

You're stuck with it for years, I'm afraid - best treatment is to drip neat glyphosate inside the hollow stems on a regular basis. Burn any parts you cut off to stop them rooting - it is illegal to put any part of this plant into the green recycling. The roots go down 9 feet, so digging it out will be next to impossible, and any tiny fragment of root dropped on the ground will grow into another plant.

As with previous comments but try hoeing off first and then spraying new growth. This should get into the plant system with better results. Can you cover the area with heavy duty black plastic for a year or two? Grass the area and the repeated cutting should see it off after a while. This also can work for bindweed.

But you need to be careful of how you disposes of the clippings, C&g. Knotweed can grow from 'cuttings' and so it is illegal to dispose of any such material except at a licenced site. See your local council for further details.

thank all bamboo soundes the best opshon becouse diging 3m down is inpossab desil did not sound write