The message
from Stefek Zaba contains these words:

I would have to agree with your KISS principle John - however, IMHO
having a
drain cock, with whatever its shortcomings, is a small price to pay
for the
speed and convenience of being able to attach a hose which can go straight
out the patio doors into an outside drain instead of spending ages
pratting
about with unstable trays full of black nasty water - however infrequently
this may be required ... YMMV )

Agreed - our previous house had an unusually intelligent version of this
idea: a drain cock in a large understairs cupboard, so the odd leakette
of sludgy gunge (if such were present) would stain only floorboards,
rather than your finest lounge carpet.

AIUI the particular problem referred to above is drop down links from
upstairs which need a drain valve on each section so a common drain cock
wouldn't work in that case.

Never did get round to running a captive length of garden hose (with
creepie-crawlie cap) through the airbrick close by - only time we had to
drain down we just hooked up the garden hose and let it snake through
hall and kitchen...

FWIW I have drains through the outside wall teed off both flow and
return at the lowest point on the circuit. Still have to use hosepipes
though as the discharge would otherwise end up in the well. Anyway being
tight I drain down into a plastic dustbin and pump back afterwards.

I also have a (gatevalved)* cross link between cold feed for the
cylinder and the feed to the hot taps so next time I have to drain down
the cylinder I won't have to wait hours while the contents syphon out.

*Or maybe just a stop cock.
--
Roger