Crystal Reports Finding if a value is in an array

Message

   I have set up an array (well actually three arrays because the list is 2500 long) of postcodes.  I now need to determine, for any address record, if that particular postcode is one of those in the array.
I created the array as
BeforeReadingRecords;
StringVar Array Postcodes1 := [
"B60 3HJ",
"B60 3HL", 
............
];
" "
and that seemed to be all right

Then I set a formula

WhileReadingRecords;
Local StringVar Inpostcode; 
StringVar Array postcode1;
Inpostcode :=  {ADDR_DET.POST_CODE};
If Inpostcode in postcode1 then "T" else "F"

which was also accepted. 

But it always returns F even when I know the postcode is in the array.

What am I doing wrong?