The earliest you can apply is 28 calendar days before 5 years of having FLR. The latest is the date of expiration of your BRP.
Interesting! I hadn't paid enough attention to what this means
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Our first FLR(M) issue date was early July 2015. Our second FLR(M) expires early August 2020. Does this mean we can apply 28 days before our early July date, so some point in June? I had naively assumed* 28 days before the August date, but what you're saying makes complete sense, and means we can get the ball rolling sooner than we thought (still 18 months away, but I'm a neurotic ducks-in-a-row person).
So, thanks for helping me with a realisation!
*assumptions are silly when it comes to visa paperwork - I should definitely know better!