Does anyone have any advice for how to extend someone's register and gain greater control and focus in the top register? I have heard lots of different methods, not many seem to work so well , any advice much appreciated.

Stephen

I believe it all starts with correct breathing technique.
Get a couple of lessons with a pro. to help you sort this out - I did and it was the single most useful hour that I've spent. You're probably thinking that at grade 8 standard there's nothing that anyone can possibly teach you about breathing (and perhaps you're right), but it's worth checking out.
Other than that, I play lots of long notes (throughout the range) and pick repertoire that I like, and want to master - not just exercises. All this, over time with patience, is working well for me. I'm playing a lot more comfortably than I was, but still a long way to go ........

Stephen,

You might want to have a look at this web page regarding a development system called 'The Balanced Embouchure'

http://www.trumpetteacher.net/welcome.html

Since I took this up (not all that long ago) my upper range has become much more solid and reliable, as well as other aspects improving. I'm confident that continuing with this will lead to further improvements including extending my range.

Get back to me if you would like to know more.

Regards,

Mark.