things have changed,
see the brief details (my opinion) on my website
http://www.baldelectrician.com/buildingstds/warrant.pdf
or the full version on the SBSA website
http://www.sbsa.gov.uk/pdfs/Electrical Guidance for Verifiers 29Mar06.pdf
If the building warrant was applied for after 1st May 2005 then these rules apply, if before then it's the same as BigJon says.
essentially the council have the discression to accept what they deem as 'reasonable enquiry'.
The above directive states the defenition of a 'competent person', which is someone who meets the standards of a member of SELECT; NICEIC or a UKAS body.
The above directive was introduced to help clarify things, so all the councils sing with the same hymn sheet as it were.
Check with the council and a
sk for a reply in writing as some councils say one thing over the phone and practise another.
If your brother in law does the work the council may ask for you to get a certifier of construction or equivalent to do a PIR on the job, it may be as cheap to get the work done by a COC.
Essentially I include COC certification in any work I carry out from start to finish in the price.
Most other firms won't look at others work, if they do a hefty premium is charged as it usually involves putting things right before certifying