What depth are the current ceiling joists?
2.5"

THe way in which the post meets the joists and the two ends of the
ceiling joists "fly past" each other would suggest there is a
supporting
wall or some other load bearing structure under there.
Nope - nothing underneath that one - just empty space! The other one
(next to the chimney breast) might be supported by part of the chimney
breast - haven't checked that yet...

There are NO load bearing walls in our flat (other than the outside
walls of course) so can anyone explain the structure of our house?

How do you know the wall are not load bearing? (a stud wall can be
load
bearing as well - it does not need to be brick / block).
There are certainly no internal brick / block walls other than the
chimney breast and all the stud walls seem to run in directions which
would offer little or no support to the ceiling joists... I could be
wrong though - no expert on such things. :-)

If there really is no load bearing wall, is there a substantial joist
at
the apex of the roof?
Not really - the joist spanning the apex looks pretty small - I've
taken some more pics which I hope will help:
http://www.pciq.co.uk/pics/apex.jpg (ignore the damp - that's on the
to-do list!).

This one gives a better 'overall view' of the loft:
http://www.pciq.co.uk/pics/loft.jpg

I don't think I'm going to have much option but to add some extra
joists to strengthen the floor in my 'storage area' - these could
potentially attach to joist hangers in the wall and rest on the main
30' beams spanning from front to back, which I presume are currently
supporting pretty much everything. The trouble there is that my floor
will now be around 10" higher which means I'll lose quite a bit of loft
space - such is life though. As you say, I don't think I've got much
option but to strengthen the supporting joists for the stringer beam
which will in-turn support the loft hatch. I certainly couldn't fit
the hatch any higher (in new joists) since I'm already at the max floor
to ceiling height (2.9m)... if that makes sense.

Still don't really understand how my ceiling's held up...

....and anyone have any ideas on the existing 'joist sized holes' in the
walls??

Cheers again!

Andy