Thanks for your suggestions, everyone - you've given me plenty to think about.
QUOTE(barry-clari @ Nov 1 2009, 11:17 PM)

Don't bite, and don't use undue pressure anywhere.
Food for thought there - I think I am biting, and probably tensed up right down to my toes!
QUOTE(clarijo @ Nov 1 2009, 11:48 PM)

Just because you can produce the note doesn't necessarily mean that you will be able to play it in your pieces straight away, so don't be disappointed if you can't! I can now get up to a high F but I don't play anything above a D in my pieces because the quality of the notes above isn't yet consistent. I'm in no hurry to get F# until the ones below are concrete either!
I think if you normally play on a 2.5, stick with it! Some people suggest a harder reed can help but I found that the position of the reed on the mouthpiece can also make a huge difference with the high notes - if you just move it up by even 1mm, you will be surprised at how it can improve the quality of the notes. I don't know if you would agree with this Barry, as I know it's also often suggested that you just take in more mouthpiece instead!
When I first started to practise the altissimo register, I was very aware of the terrible sqwawking sounds I was making and I think this prevented me from giving the notes full breath support. Try not to change your embouchure or posture and just breathe confidently down the clarinet - EXPECT that note to come and don't be scared of it!
Yes, I'm beginning to see that it will be a long haul. I've done some practice this evening but if anything I'm getting worse! Some days just are like that, I suppose.
I know what you mean about the terrible squawking sounds - they're discouraging, and I'm painfully aware that everyone else in the house has to listen to them. Thanks for mentioning about taking in more mouthpiece - my teacher did mention that, but I'd forgotten.
I spent a little while just playing the high notes slowly tonight, and afterwards I found that when I played in the chalumeau register the notes were coming out high so I must have been doing something right, though at the wrong time, if you see what i mean.
By the bye - is it just me, or is the B flat on the middle line of the treble clef a bit of a duff note on the clarinet? It always sounds out of tune.