any time you are using comma separated lists in your tables, red alarm bels should ring. Comma separated lists are a bad database structure, and will not work properly for what you want to do.
you should be using a linking table to create the many to many relationship of post codes
tableID - Primary Key
postcodeID - Primary Key
itempostcodeID - Primary Key
itempostcodeTableID - relates to maintable.tableID
itempostcodePostcodeID - relates to postcodes.postcodeID
this way you store a record to assign each post code to the records in the main table.
you will need to use a join query on the results page and then in the key word, you can use equals instead of includes