Section 21 (1b ). How to serve as part of tenancy

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I am a fairly new landlord and still a bit wet behind the ears, so please bear with me.

I have tenant whom is in a 2 months arrears situation because of ill health and loss of job. She is trying her best to pay her rent and we are sorting it together with the benefits office( has been ideal tenant for over a year ). I was not aware that I can serve a section 21 at anytime and would like to do so now to speed up possession should the need arise.

How do I serve this to the tenant without putting the fear of imminent eviction into her ? ( the wording will make her think this )

How do I word it to her ?

Her AST finishes 1 October 2011.

Many thanks for any advice.

please help