PlantTracker

http://planttracker.naturelocator.org/

The PlantTracker project is a collaboration between the Environment Agency, the NatureLocator team at the University of Bristol and the Centre for Ecology and Hydrology. The project kindly provides us data with locations of himalayan balsam for our Known presence map. You can also report and view maps of other invasive non-native plant species on their website.

Non native species secretariat

https://secure.fera.defra.gov.uk/nonnativespecies/home/index.cfm

This is an excellent source of information. It has a map showing current projects going on around the UK, making it easy to find projects near you, or of a similar nature to yours. This also lists the contacts for each project, so you can get in touch and get ideas.
It also has a very good set of identification sheets, and photographs.

The Cornwall Knotweed Forum

http://www.cornwall.gov.uk/default.aspx?page=13835

This page has a leaflet that can be printed off, as well as a useful short guide to control, and pictures. The information on Japanese Knotweed is a little more in-depth than the Himalayan Balsam information, but it is still well wroth checking out.

The Environment Agency

http://www.environment-agency.gov.uk/homeandleisure/wildlife/31350.aspx

This webpage gives you information on how to control invasive plant species, and also gives you the link you need to applying for permission to use herbicides near water. Please note you need to apply at least 3 weeks before spraying.

Countryside agencies

The countryside agencies in the UK may be aware of invasive species control projects going on in your local area, and it may be worth contacting them.

The Countryside Council for Wales
Natural England
Scottish Natural Heritage
Northern Ireland Environment Agency

Invasive Species Ireland

http://invasivespeciesireland.com

This website gives some advice on managing invasive species, what you can do to help minimise the spread, and also gives you an option to map the presence of invasives in Ireland.