http://designatedsites.naturalengland.org.uk/PDFsForWeb/Citation/.pdf
Use this template to add the Natural England SSSI citation link to your ref. If the website changes, a single update on the template can keep all the links up to date (so long as they don't change the id numbers).
To use:
{{sssi link|id number}}
Example:
{{cite web | title=Bonemills Hollow SSSI citation, Cambs.| url={{sssi link|1000112}} }}
The template simply expands the id number to become the full url, so can be used wherever you would normally add the url itself.