Belle,
The FLR(M) should be completed as soon as you can following your marriage.
There's really no need to wait until the Fiancee visa is near to expiry.

The FLR(M) form used is for two purposes, that of application for Further Leave to Remain and also for application of Biometric Residence Permit.

You MUST complete for both.
At section 9 of the FLR(M) form it states:-
It is mandatory to complete this section as required. Please note that this application will be invalid if you do not do so.
Good luck