I've recently done the electrical wiring in my workshop, which is a
separate room tacked on to the end of a detached garage; so what this
entailed was simply adding on to an existing circuit in the garage (a
single radial socket) and extending it into a ring with 7 sockets. I
understand that you can extend an existing ring without having to put
your head above the Part P parapet?; I'm not sure whether what I've done
would qualify under that (since there wasn't a ring there as such), not
that I'm unduly bothered either way but am curious?

It also got me thinking... if the new workshop extension wiring did
qualify as exempt from Part P, what would happen if I went back a couple
of years later to sort out the garage wiring, ie to rewire that part of
the ring? That should then also be exempted under the same rule,
surely? - despite that fact in fact the entire circuit would have been
installed by a non-Part-P-qualified person with no Building Notice...
the old "my broom is 50 years old; but it's had 15 new heads and 10 new
handles" thing. Is that right?

(As it happens the existing garage radial was indeed installed by yours
truly; but in good old red and black cable, years before Part P ever
raised its ugly head...!)

David