Japanese Knotweed in neighbour's garden

[MikeUniformMike]

My NDN has lots of weeds in their garden. Some are creeping into my garden. Both ND and their NDN have Japanese Knotweed. I cleared it from my garden but their's is the size of a tree.
Is there anything I can do about it?
I have tried the Environment Agency, which was of no use.
I have also given next door se weedkiller in the past and that made no difference ( I think they just binned it)

Next door's next door are tenants.

Just to clarify: se weedkiller should read some weedkiller.
It seems to be in the gardens of several houses going down the street.
Next door has lots of weeds including several clumps of JK.
Other weeds (not JK yet) are creeping into my garden.

Thanks for reading.
MUM

Japanese knotweed is a notifiable weed as per this website:

www.gov.uk/guidance/prevent-the-spread-of-harmful-invasive-and-non-native-plants

Your next door neighbour should be taking this seriously as you could get an enforcement notice to force them to take action. It's expensive and companies specialise in it but it is possible to treat JK. If it's left then people in your street will have trouble selling their houses. (One house I knew had to be sold at auction as it couldn't be sold through an Estate Agents).

Thanks. I have read that page but am none the wiser really. I got rid of it in my garden by repeated and vigilant use of glyphosate but there is JK about a metre from my garden. It took about 6 years. My NDN is abroad but someone is staying there now. Their NDN has bonfires in the garden nearly every day but has a massive clump.
If I tried to sell my house I probably couldn't.
How can I get my neighbours aware that they should do something about it? As it is, I think all I can do is wait for it to invade my garden and then sue. The environment agency will only act if it is agri/horticultural land.

This must be very frustrating. I think if it spreads to your property from theirs then your neighbour is liable? But I suppose that leaves you waiting for it to happen rather than being able to prevent it :( - sorry I'm not more help!
Ive fallen in love with a property which has knotweed nearby, not on the boundary but in an access lane next to the property which connects to a no-mans land sort of area. Everyone is saying not to go near it with a barge pole as no idea how bad the JK is or how much it'll cost to remove etc. EA suggested council might be liable - but I doubt it!

It is not the huge issue it is sometimes made out to be. here is the advice from RICS, the Royal Institution of Chartered Surveyors.
Quote from this:
If treatment is deemed necessary in cases where Japanese Knotweed is present in the grounds but no damage has been caused to the property itself, the sole expense may be the cost of the treatment itself. In some circumstances, this may have no adverse affect on value.