s161 TCGA 1992

I have read the legislation, referred to various books and even looked on the HMRC website, but can not find the answer.  So I am hoping someone on AW can help?

All sources refer to making an election for s161 to apply but does this have a be a formal written election submtted to HMRC or can it be made on the relevant CT600.  If on the CT600 which box/boxes need to be completed because I have spent the previous 2 hours trying to find where to enter this on Iris business tax !

Thanks in advance