I think they're a brilliant idea - but I don't have them at the moment,
and
can't think of a way of fitting them without draining the rads in some
other
way to start with!

Well, I did the whole lot at the same time, so the whole system was drained
down. No need to drain individual radiators then.

Is it fair to assume that they're longer than standard tails - and so
require some pipework adjustments if they're retro-fitted?

Nope. Mine were identical to non-draincock types in terms of geometry. The
main problem with them is that they stick out the other way more, which some
(i.e. those who aren't responsible for removing radiators) may find
aesthetically challenging.

Christian.