Forgot to mention that I entered <?php ECHO($WA_Sql);?> at the end of the update to see if I was getting errors and I did get this:
UPDATE `tbl_forumPOSTS` SET `fld_fRELPYCOUNT` = 0, `fld_fREPLYDATE` = 'NOW()' WHERE `fld_fID`=5
Warning: Cannot modify header information - headers already sent by (output started at /Users/matthewstuart/Sites/ktc/forum_reply.php:347) in /Users/matthewstuart/Sites/ktc/forum_reply.php on line 385
I have since moved the ECHO and the error has disappeared, but I am still using the same code, so I am not sure if the ECHO itself was causing the headers error! So the lines have changed a little, and the ECHO can now be found just before the DOCTYPE declaration.