Here are some websites you may find helpful. I hope to update this regularly.
LAST UPDATE 2/8/2015

The NHS Hoarders information page should be you're  first place to visit and share with the hoarder.

 I am excited about this UK based website helpforhoarders, I love this website as it has plenty of resources.

Childrenofhoarders a support group

OCD-UK  a charity packed with resources, advice, research, where to get treatment and much, much more on OCD.

http://www.yvettewrites.com/ top tips on how to declutter and stay organised