You're taking quite some risk by cutting it so fine.

You need to get fully prepared asap with your application for FLR(M) complete with all documents.
Once married and you have some marriage certificates get the application mailed off to The Home Office.

The Home Office MUST receive you application before your visa expires.

If you really need more time you can seek an extension to your fiancee visa but you need to evidence GOOD reason why you need the extension and GOOD evidence when the marriage will take place. Don't rely on being granted though and push through with all plans to submit the FLR(M) immediately following the wedding. This should be considered a possible back-up only