Hello! I was issued my visa on March 17th of this year. I entered the U.K on May 13th. I am a very paranoid person, and I like to be prepared ahead of time. So when my visa expires on December 17, 2017, I want to be more than prepared.
Just for the record, you can apply for FLR(M) when you reach 30 months in the UK - you don't have to wait until the visa expires.
You will reach 30 months on November 13th 2017, so you can apply any time from 28 days before then (about 16th October) and the expiry date of your visa, though I wouldn't apply too early - as close to Nov 13th as you can
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Now I know I need proof that we will together, the only thing I have is a few mailed things, like letters from the HM regarding my NIN. I am also on my husbands bank account. We are moving in September so on that tenancy agreement we will both be on it. I'd just like to know what else I need to be able to get to extended Visa (aka document list).
The visa requirements are pretty much exactly the same as for your spousal visa, so you include all the same documents as you did for the spousal visa, but instead of showing things like trips to see each other and email correspondence while apart, you show you have been living together - by sending the 2 years of mail addressed to both of you at the same address.
You only need to cover the previous 2 years of co-habitation - so although it's a good idea to save all your mail, you will only need to include mail you have received between November 2015 and November 2017.
You'll need 6 documents in each name, at the same address as each other and they need to be from at least 3 sources, spread approximately every 4 months of the 2 years - you can either have jointly addressed documents or separately-addressed... so 6 joint, or 12 separate, or a combination of both.
The FLR(M) application form lists all the details about the 6 documents and also lists which types of documents are accepted:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/429312/FLR_M__04-15.pdf