Sorry, I had tried responding by email, but that was bounced. I recompiled this morning and now it works. No changes to the code at all. Don't you just love computers?

Many thanks for all you ideas and the quick response.

Anyway, for completeness here is the spec of components etc 

Delphi XE2, FireDAC TADQuery, Firebird 2.5.2 with fbClient.dll - all fully updated

to remove any variability with passing the variable to routine, the email address has been hard coded. So code is:

Database

record 1 : email = pam.o'neill@...

record 2 :  email = pam.oneill@...

Code
resourcestring

sql = 'SELECT * FROM PERSON WHERE EMAIL = :EMAIL';

sql1 = 'SELECT * FROM PERSON WHERE EMAIL = ''%s''';
...
begin
emailvar := 'pam.o''neill@...';

emailvar1 := 'pam.oneill@...'; 

AQuery := TADQuery.Create(nil);

try

  AQuery.SQL.Text := sql;

  AQuery.ParamByName('EMAIL').AsString := emailvar;

  AQuery.Open;

  result := not AQuery.IsEmpty;

  AQuery.Close;

finally

  AQuery.Free;

end;

end;