Section 21 Eviction Notice

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I hope you can help me with some advice please, the siuation is this:-

I have a tenant who has been living at one of my properties for over 4 years, in full time employment and never been in arrears.

However, she has now gone back to University, and this first month of Uni, has only paid me half the rent. I have asked, emailed, phoned etc, but to no avail, she has become uncontactable and I am now thinking of sending her a Section 21, so I have this in place if she continues not to pay in full.

My questions are:-

1) As the AST is now a roll on month on month agreement, which Secion 21 do I need (A or B?).

2) Also, if she does pay, I will keep an eye on whether or not to evict her. However, am I able to issue her with a Section 21 in case I ever need to enforce her to leave and therefore I will have already given her the mandatory two months notice?

3) Does anyone know about the Accelerated Eviction proceedure at all, and whether or not this would be more feasible than going down the Section 21 road, or is this in conjunction with the Accelerated Eviction Proceedure?

I don't want to issue her with a Section 8 for Rent Arrears as I understand if she pays anything whatsoever towards these arrears, it takes it back to square one, and I would have to start again from scratch losing valuable rentable time.

Any help here would be massively appreciated!!!

Huge thanks!!