If they are removing knotweed then it has to be disposed of in a certain way. Lewisham doesn't have any information on its website but http://www.knotweed-uk.com/Knotweed-and-The-Law.htm says the following:

Environmental Protection Act 1990

Japanese Knotweed is classed as ‘controlled waste’ and as such must be disposed of safely at a licensed landfill site according to the Environmental Protection Act (Duty of Care) Regulations 1991. Soil containing rhizome material can be regarded as contaminated and, if taken off a site, must be disposed of at a suitably licensed landfill site and buried to a depth of at least 5 metres.

Section 34 of the EPA imposes a duty of care on persons who produce, import, dispose of, or treat controlled wastes. The movement off site of controlled waste must be covered by a waste transfer notes. The transfer notes must be completed and signed, giving a written description of the waste and a waste code. This description must be comprehensive enough to allow the receiver of the waste to handle it in accordance with their own duty of care. These provisions are set up in the Environmental Protection (Duty of Care) Regulations 1991.

Section 33 of the EPA states that it is an offence to deposit, treat, keep or dispose of controlled waste with out a licence. There are exemptions to waste management licence's stated in the Waste Management Licensing Regulations 1994. The Environment Agency Code of Practice 2006 states in accordance with their Enforcement and Prosecution Policy, failure to have a waste management licence or permit, when dealing with the knotweed growth on site, would not normally be prosecuted if the Agency’s Code is followed.

An offence under the Wildlife and Countryside Act can result in a criminal prosecution. An infringement under the Environmental Protection Act can result in enforcement action being taken by the Environment Agency which can result in an unlimited fine. You can also be held liable for costs incurred from the spread of Knotweed into adjacent properties and for the disposal of infested soil off site during development which later leads to the spread of Knotweed onto another site.

As well as the two items of legislation outlined above, third party litigation for damages may be sought by adjacent landowners when Japanese knotweed is allowed to spread onto other property.

Are you sure it's Japanese knotweed they are moving to the other site?