Graham,
When I told a stove supplier I intended to do a self-install and get the council to sign it off later, he told me that the council probably wouldn't be interested in doing that (they weren't, and confirmed that for our job, no building warrant was needed as long as we complied with the codes), and also that HETAS don't have the same status as a statutory body in Scotland that they do in England.
We ended up doing two chimney linings and stove installs ourselves (recommend the
stove fitters manual for guidance and
hotline chimneys for parts). Both stoves were straightforward to install, and we saved 60% compared with what the stove shops quoted.
The building regs for stoves are actually quite straightforward too, so it's easy enough to read them to ascertain what you need to do to comply, and if a building warrant (or registered install) is required or not. If you don't want the hassle of doing that of course, don't chance it, and use a registered installer.
Also if DIY is not for you, is there a local chimney sweep you could use?