It’s very unlikely that Japanese Knotweed has self seeded in your garden. Even when they do produce seed it rarely germinates or survives the first frosts. Japanese Knotweed spreads in the UK through its rhizomes and pieces of broken stem. If you have spotted fresh shoots in your garden they may be coming up from a rhizome which can reach 7 metres from the parent plant. If you can see a Japanese Knotweed plant growing nearby this is most likely the cause.
If you do have Japanese Knotweed it is best to get rid of it as soon as possible. The best method is to cut the stems and insert a glyphosate based herbicide into the hollow stumps - you’ll probably need to do several treatments to properly eradicate it! Dispose of any cuttings at a licensed landfill site as it is illegal to fly-tip or spread Japanese Knotweed. We would recommend talking to your local council as they will have information and advice about safely disposing of Japanese Knotweed locally.