So far so good; where I am unclear is what the rules are in between
the above scenarios. What about if I buy a property - equally old and
not fulfilling current regs - in order to do a refurb and sell on or
let out?

Anything you change must be done to current standards.

i.e. an extension would require the full shebang to modern standards. A
rewire would be required to modern standards. Major alterations to part of
the dwelling would require you to install mains smoke alarms in those parts.
Any new walls would have to be insulated to the required u-Values. A new
heating system would have to comply with Part L. etc.

Rules for renting (particularly houses of multiple occupation) may be
different, requiring changes to bring stuff up to date, even if you weren't
intending to touch them. This is particularly the case when it comes to fire
safety.

Does it depend on the extent of the refurbishment (eg bit of wallpaper
and paint versus major structural alterations)?

Yes. If you wallpaper an external wall, you don't have to do anything. If
you rebuild the wall from scratch, you'll have to insulate it, unless you
can pass it off as a "repair".

Christian.