That's gret thanks , but what or who are MAF?

ministry for agriculture , fisheries and food. i think.

If you only have a bit and there isn't loads of it on the other side of the fence you can control it largely by cutting off the stems and pouring deep root weedkiller into them.

Before you get involved with MAFF I would suggest seeking advice from the local office of the Environment Agency, there will be one in south Lincs.

If it's on your land, unfortunately, you will be responsible for the cost of its eradication, and it can be rather high. Don't try to do it by yourself, though, unless it's a very small amount. I believe cutting it down to the ground and using very strong weedkiller on the new shoots that regrow is the idea, but it could take several applications. Nasty problem to have.

We have Japanese knotweed growing by our pond and for the past ten years we have simply chopped it right down as soon as it grows in spring, and kept at it during the season. It is growing next to a clump of vigorous bamboo which maybe reduces the problem. It doesn't appear to be a problem now and has certainly not spread.
It is important to bruise the stems before you spray it with something like Roundup (strong solution) and you will need to repeat the treatment several times.
Let it grow again to a reasonable size (6 ins to a foot) before repeating the treatment, as Roundup works best if it can be absorbed over a good area of plant.
I am surprised if the control of it is entirely down to MAFF now as there are still huge clumps of this stuff growing across the UK. It's important it isn't spread into the wider environment though so DON'T put any remains of the plant, however small, in compost or even in your green bin.

Stergram and Bertiefox have got it. Cut the plant down to about 6 inches. Push a screwdriver down the stems and then using a pipette, pour a strong solution of Roundup into the hollow stem. Make sure that you dispose in your general rubbish or burn if possible.