Hi
We get asked to advise and report on Japanese knotweed fairly often, including by people looking to buy a house where its been found in the garden.
the estate agents (who?!) are indeed wrong to say its not a problem as its a notifiable weed and there are various legal implications
if you are sure that its Japanese knotweed, then unless your heart is set on the house I would walk away as being easiest
of course it may not be japanese knotweed as there are similar looking plants - so if you do want to go ahead, then get this checked by a horticulturalist and a proper report written up, then you'll be covered for mortgage and insurance purposes
this is what we can offer and there are other trained people out there too
http://plewsgardendesign.co.uk/service/garden-sos/