There are always options:
Contract a specialist to remove the knotweed.
Implement a lower-cost treatment plan over the long-term to contain the plant.
Contact a broker for professional guidance following The Property Care Association's ruling that knotweed should be treated as any other type of property issue per the Invasive Weed Control Group 2012.
These same restrictions usually apply to buy to let investments and residential mortgages, with similar policies from lenders in either situation.