I’ve solved it by creating a Custom Meta Field filter using the following options:
-01-::Jan|-02::Feb|-03-::Mar|-04-::Apr|-05-::May|-06-::Jun|-07-::Jul|-08-::Aug|-09-::Sep|-10-::Oct|-11-::Nov|-12-::Dec
The “-mm-” in each part matches the month stored in the database. Pretty simple really but this works perfectly.