My only concern here is that the entire weight of the ceiling, the
loft ladder (and people on the loft ladder), the contents of the loft
and potentially the king post would be supported by the bits marked
'A' on plan.gif. Should I extend the 'Blue' supports out further (to
the next set of joists either way) to spread the load?

Personally, I would say that you would need to replace your "blue" beams
with sufficiently strong ones hung from structural brickwork at both ends.
Simply tying the cut joists together when they are already worse than
marginal probably won't do. An alternative would be to strengthen the next
uncut beam either side to hang your "blue" beams off, either using a very
deep wooden beam or a steel beam sufficiently protected against fire, which
may fit within the existing joist height.

Either get a structural engineer to design the thing, or have a play with
SuperBeam, if you've got enough mechanical knowledge to make safe use of it.

Personally, if I was doing structural alterations of this sort, I would
design a whole new structure based on standard floor loadings. That way you
can be sure that (a) you're laughing if you decide to convert the loft and
(b) you can use the loft for much heavier storage without breaking the
ceiling.

Christian.