The message is encrypted with

>! a [Playfair][pf] cipher. This is hinted at by "Fair play swapped will bring you success". They key used to create the table is "Mr Mister" and the table is   
>!    
>! `M R I S T`  
>! `E A B C D`   
>! `F G H K L`  
>! `N O P Q U`  
>! `V W X Y Z`  

The plain message reads:

>! The man stared out on the brilliant sea,  
>! As you bent down upon your knee.  
>! Whilst eating the pea, which was rather nice,  
>! He declared it "to be" as he threw the dice.  
>! "Don't owe me any money!", the man did cry,  
>! In the supermarket, a person wandered by.  
>! "Are you coming tonight?" questioned Dee,   
>! The man was angry, yet quiet was he.

The location of the murder weapon is hidden in this text:

>! The poem contains several words that are homophones of letter names:  
>!    
>! The man stared out on the brilliant **C**,  
>! As **U** bent down upon your knee.  
>! Whilst eating the **P**, which was rather nice,  
>! He declared it "to **B**" as he threw the dice.  
>! "Don't **O** me any money!", the man did cry,  
>! In the supermarket, **A** person wandered by.  
>! "**R** you coming tonight?" questioned **D**,   
>! The man was angry, yet quiet was he.

The weapon is ...

>! inside the **CUPBOARD**.



  [pf]: https://en.wikipedia.org/wiki/Playfair_cipher