Dennis,
Here is a pic of the blocks. The wall is 7' high. The blocks are 8 x 8
x 9. One side is vertical, the other is tapered. The top course of
blocks comprises bell shaped copings
http://lh4.ggpht.com/tonyjeffs2/SQh7...y%20blocks.jpg

Jim,
I did check with planning.The planning requirements changed at the
beginning of october. We're allowed to take it outwards by 3 m without
planning permission, but we want to take it out 4 metres so do need
consent. The total floor area is 12m.
I think you have a point though about the whole thing falling in on my
head - if I don't get the join with the garden wall right, that could
be a source of endless damp problems!! :-/

Here's a mockup of roughly what the conservatory will be like, but
without that silly peak on the roof.
http://lh3.ggpht.com/tonyjeffs2/SL_u...ory%20copy.jpg

Tony