UK fiance visa: Sponsor's documents.

Hello, everybody.

I have a question for you (apologies if it's long winded). My fiancee and I will be applying for a visa in the next couple of weeks. She will be moving from America to the UK. We've pretty much got all of the evidence we need, but there's one thing that's concerning me.

I rent the flat that I'm living in and have lived here for 2 and a half years now. However, I am now on a month-to-month tenancy agreement, as opposed to a fixed-term agreement. I've read a few times that sponsors have included a tenancy agreement in with their documents as evidence. Will being on a month-to-month contract affect our application? I have a letter from my landlord confirming that my fiancee is welcome to move in upon her arrival and that I've lived in the flat for as long as I say I have. Also, the letting company are sending me a letter of confirmation (similar to what I got from the landlord) on headed notepaper.

So, the long and short is: when applying, does the sponsor need to be under a fixed-term tenancy contract when renting a home?

See also, Guide to supporting documents: settlement (click).