tenant won't move out

Possession - Section 8 and Section 21

I have a tenant who it is proving very hard to evict and i am unsure of my options. I have spoken to 5 different lawyers and it seems I have to keep going back to court but I am concerned that each time she scrapes through on a technicality she is getting the message that she can stay as long as she likes. She has no reason to move out as the rent is below rental market rate and housing benefit are paying it anyway. What are my options?

I signed an Assured Tenancy Agreement with her and her boyfriend in 2008. in 2010 her boyfriend moved out and I gave them back their deposit as she was going to stay and give me the deposit herself. Then I would have signed a new tenancy agreement with her. She was now a single woman paying the rent on her own, so I understood when she said she didn't have all the deposit so I agreed for her to pay it in instalments and that we would continue on the previous tenancy agreement on a month to month basis. Months later she had only given me £400 of the £1095 deposit so I just put that in the same deposit protection scheme based on the same tenancy agreement from 2008, when i had provided her with the prescribed information.

I served her a section 21 notice over 5 months ago and she has not moved out. My lawyer told me that a lawyer advising her said that as I did not send the prescribed information when I put the £400 deposit back in the scheme, that the section 21 is null and void.

I do not know what I am meant to do now?

if I give her back her deposit, that means I have no protection. (I am happy to do this as £400 is worth much anyway.) But does this still mean I can use this present section 21 notice? Or should I give her back her deposit and start again with another section 21?

But what if I release the deposit, but she doesn't claim it? as she has been told by a lawyer that that means I can't serve a section 21 notice?

Or do I go to court based on this section 21 notice in the hope that the judge recognises that we have done everything according to the original tenancy agreement which includes the prescribed info: I have never raised the rent, Housing benefit recognise that tenancy agreement, the judge who presided over the arrears hearing also recognised it.

My lawyer says this is a risk as it is up to the judge I get as to what happens in court but he has not offered an alternative that can work.

Or should I offer her a new tenancy agreement and raise the rent (she is not bothered by paying more, (as she said "money is no problem" - it is for me) do everything correctly (as far as I know) and do another section 21 in 6 months time.

Will I ever be able to evict her? What are my options?