Selling and buying a property can be really exciting and quite stressful.
My advise would be if you suspect there is Japanese knotweed in a garden you intend to buy, send a photo to a professional company (preferably a member of the PCA) and confirm with them if the plant is. If it is what is the cost of controlling/removing the plant depending upon what you would like to use the area where the plant is for.
If you are selling - being upfront as soon as possible really does help. The potential purchaser will then be in a clearer view and saves any last minute panics and withdrawing their offers.
Don't cut the plant down and pretend it does't exist, otherwise it tends to make the plant bushier when it regrows and it tends to regrow quite quickly, potentially causing more issues down the line.
If you have any concerns Longfield Knotweed Solutions can provide a five or ten year management plan to control the plant, which is suitable for mortgage lenders and provides everyone with a peace of mind.