The Law

Due to the destructive characteristics of Japanese Knotweed, strong legislation is in place to control the magnitude of the problem.

5 common legal issues with regards to Japanese Knotweed:

You can be sued (under laws relating to private nuisance under common law) for costs and damages if you allow Japanese Knotweed to spread from your property onto that of an adjacent landowner.
It is an offence to cause the spread of Japanese Knotweed under Section 14(2) of the Wildlife and Countryside Act 1981 (WCA 1981)
Fine of £5,000 and/or 6 months imprisonment, or 2 years and/or an unlimited fine on indictment.
The Environmental Protection Act (EPA) 1990, Duty of Care Regulations 1991 state that cut Japanese Knotweed material and soil containing rhizomes must be disposed of as controlled waste.
It’s not illegal to have it on your land
It’s not a notifiable weed

For a full description of all Japanese Knotweed legislation, please refer to the Environment Agency ‘knotweed code of practice’.
http://www.environment-agency.gov.uk/static/documents/Leisure/japnkot_1_a_1463028.pdf