Getting the name of the timezone, adjusted for daylight saving

Running 8.4.6 hosted on Linux, if I do this...

SELECT to_char('2011-03-01 12:00' AT TIME ZONE 'GMT0BST', 'HH24:MI TZ');
  to_char
---------
  12:00
(1 row)

..I don't get a timezone- I can live with that.

If I do this...

SELECT to_char(('2011-03-01 12:00' AT TIME ZONE 'GMT0BST')::TIMESTAMP
WITH TIME ZONE, 'HH24:MI TZ');
   to_char
-----------
  12:00 GMT
(1 row)

..then I get the GMT time with a timezone 'GMT'- that's what I want.

But if I do this...

SELECT to_char(('2011-04-01 12:00' AT TIME ZONE 'GMT0BST')::TIMESTAMP
WITH TIME ZONE, 'HH24:MI TZ');
   to_char
-----------
  13:00 GMT
(1 row)

..then I get the time corrected for daylight saving- which is what I
want- but the timezone doesn't indicate that daylight saving has been
applied.

In the general case, how can I get TZ indicating whether daylight saving
is in effect? Alternatively, is there a flag I can retrieve indicating
that a timestamp has been corrected for DST so that I can select an
alternative name for display?

--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]