By default the store pickup module only works with postcodes that start with two letters. However local to us is Glasgow which only uses a single letter (G).
The following edit will get the spu module to check and see if the customers postcode starts with a letter followed by a number (e.g. G8), and if so will only take the G to be checked against the array of allowed postcodes. Otherwise it will take the usual two letters (e.g. ML) and check them.
Find around line 67;
$cust_cp = substr($codep['entry_postcode'], 0, 2);
$cust_cp = ((preg_match("/^[A-Z][0-9]/", $codep['entry_postcode'])) ? substr($codep['entry_postcode'], 0, 1) : substr($codep['entry_postcode'], 0, 2));