If these are the only options they offer, they are failing in their duty to provide inspections themselves. (even if they subcontract this out)
Contact labc services
http://www.labc-services.co.uk/contactus/default.asp?editorial_id=13869
and see what they reckon.
It does not have to be the householder who organises someone to provide the certificate.
By the way , I beleive full blown NAPIT members are can certify the work of others, not to sign the box as "installer" of course, that would be fraud, but to sign off the 'inspection and testing' part of the form, assuming they do the electrical tests, and they pass.
The DIYer himself signs the section for "design" and "installation", assuming he has done them.
It may be this route your council intended to describe, and they worded it wrong.
BUT they still have a duty to arrange a visit from such an inspector if the housholder does not know one, even if its not one of their staff, but someone they contract in for the day - and only some of his fee can they pass on to you..