The following links might ask you to submit your name and email address (in order to access the documents) but it's perfectly safe to do so (or you can simply make something up).
A 'normal' notice, ending the tenancy (as permitted under the tenancy agreement) without any indication of fault, is issued under Section 21 of the Housing Act 1988, using this form:
http://www.propertyme...ion21-notice-form.pdf
To end a tenancy on the grounds of 'breach of contract', you need to apply to a court for an eviction notice. You must advise the tenant of your intention to do so, using this form under Section 8 of the Act:
http://www.propertyme...rces/Section8form.pdf
When completing that form, you'll need to cite the relevant ground(s) for seeking eviction, which must be quoted exactly from here:
http://www.legislatio...tusWarningSubSections
Chris