The message is encrypted with
a Playfair 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.
Note:
There is an "as", a homophone of S, that isn't accounted for in "As you bent ..."