When preparing a statement of reasons, you are not required to include any information which may be exempt under the Act. You may also apply the 'refusal to confirm or deny provision" where it is appropriate to do so

Is the right to reasons retrospective?

No. The right to reasons set out in section 18 applies to acts of public bodies made on or after commencement of the FOI Act i.e. on or after 21 April, 1998.