A new (free) app for smartphones (iPhone and Android) has recently appeared, called PlantTracker. It has been developed by the Environment Agency, Bristol University and the Centre for Ecology and Hydrology to help plot the distribution of 15 highly invasive non-native plants in the UK, including the very well-known (?notorious) species like Japanese Knotweed, Himalayan Balsam, Giant Hogweed and Rhododendron, but also some that may be less familiar but no less potentially damaging like Water Fern, Floating Pennywort and Piri-Piri Burr.
There are images and descriptions of all 15 species and, when you encounter one of them, you can get the location using your phone's inbuilt GPS (you don't need a phone or internet connection for this, only the automatic satellite link), add details of how much there is and any further relevant information like habitat, take a photograph if you feel confirmation is needed, and then, when you've got the necessary connection, you can e-mail the information to the organisers. You can also enter records manually on their website at: planttracker.naturelocator.org.
Here's a Himalayan Balsam plant I found today. I shall not only be submitting the location, but I shall probably talk to the landowner about getting rid of it. There were quite a few more plants and the potential for them to spread where they are is high.