Dominic,
There are a couple of things, check-list if you like, that really helped me get a handle on low notes and be more consistent.
As mentioned previously dropping half a reed strength may be a good idea (at least until you get more consistency)
Embouchure is important down low, the muscles at the corners of the mouth need to be firm. If you form an exaggerated TWOOOO and then EEEEE this helps you feel the right muscles. Also do a broad closed mouth smile, then try and force the corners of you mouth firmly down. Now your smirking. Around the mouthpiece this gives a nice sideways pressure on the mouthpiece and reduces top and bottom pressure. But try not top drop the lower jaw.
Understand the concept of 'warm air'. This is the breathing from the bottom of the lungs supported by the diaphragm. It is the breath you use when you mist up a mirror or window pane, not the breath you use when you blow out a candle.
Finally your throat must be very open for low notes. Any tension in your larynx will stop the note from coming. Again think about misting up a window on a frosty morning or say an aspirated 'HOT'.
If you think about these points when practising 'long tones' you should find that your low notes improve very rapidly, both being able to hit them cleanly on demand and their tonal quality. Check out this page for a really good 'long tone' method
http://www-cs.canisius.edu/~bucheger/SaxPages1.html
and Petes pages on Tone Production.
Cheers,
Al
