Cheers for the replies! Comments below...

If your joists are only a bit bigger than 2 x 2, they're only capable
of
holding themselves plus the ceiling up - and I certainly wouldn't
want to
store anything other than empty cardboard boxes on top of them!

Bugger. A cinema room with sofas, people and various other heavy stuff
would be out of the question then? :-(

Some of your pictures seem to suggest that the joists are not all at
the
same level - implying that the ceilings slope. Is this the case?
--
Cheers,
Set Square

No - they're all at the same level - must just be my dodgy pics. To
summarise, the loft area is 20' x 30' and there are two beams
(effectively splitting the loft into three 30' x 6.6' sections) with
king posts running from back to front (30' stretches) - these are 2x7"
beams joined together in the middle somehow (double sided sticky
tape?). The ceiling joists run left to right (20' streches - broken up
by the chimney in the middle) - these are 2.5 x 2.25" joists with a gap
of 12" between each. The loft feels VERY sturdy to walk on (nothing
really happens when you jump up and down on the joists) but I still
don't really understand how?!?

Anyhoo... I've just spoken to a structural engineer and he reckons the
ceiling joists will be held by the king post beams somehow - don't
quite understand this. Anyway, getting him to come out and have a
look, which is probably a good idea.

Any more advice would be greatly appreciated though! :-)
Cheers again,

Andy