It appears to have spread to the other side of Dartmouth Rd, Devonshire Rd and is even into the rear gardens of St David's Rd. I'd try working with those neighbours if possible.

All waste containing Japanese knotweed comes under the control of Part II of the Environmental Protection Act 1990 and is classified as a controlled substance. If you follow the links I gave, you will be much better informed.

Treating Japanese knotweed early and effectively can significantly reduce the chance of it growing again. You should agree and implement a treatment plan as soon as possible. It is essential that a competent and qualified person carries out the herbicide treatment. Contractors must have the appropriate National Proficiency Tests Council (NPTC) certification. They must carefully follow the instructions on the herbicide label. You can only use certain herbicides in or near water, and you need approval from the Environment Agency before you can use these.
The most effective time to apply glyphosate is from July to September (or before cold weather causes leaves to discolour and fall). Spring treatment is acceptable, but less effective. Triclopyr, picloram and 2,4-D amine can be used throughout the growing season. You should avoid the flowering period to protect bees and other pollinating insects. The majority of herbicides are not effective during the winter dormant stage because they require living foliage to take up the active ingredient. An exception to this rule is picloram, which can be applied as a soil treatment