So I've got unicode set up and seeming to work in most things, I've got LANG set to en_GB.UTF-8. vim and lynx both show © as the copyright symbol but less displays the hex code in <>'s and inverse - ie <A9> with the background and font colours inverted, while stdout just displays them as a block. Can anyone suggest how to fix this?

At a guess, you have the wrong keyboard set?

Start reading and checking. consoles in X and on Alt_Fn use different software. Anything with a script can set an environment, and some do. Look up /etc/environment. You probably won't have one. Then consoles and X have separate keyboard layouts, etc. Enjoy!
A good way into this is to do a linuxfromscratch which takes you through some of the issues, as you build from scratch. Hacking a sophisticated distro to suit your tastes can be difficult in the extreme.

Keyboard is correct, and all this is in a virtual terminal (Alt-Fn). For example:

% echo $'\xA9'
prints a block (A9 being the hex value of '©')

% echo $'\xA9' > tmp
% less tmp
shows
<A9>
in inverted colour

% vim tmp
opens the file in vim showing the '©', and

% lynx tmp
opens the file in lynx, again showing the copyright symbol. So lynx and vim are properly displaying utf-8 characters, but less (and more and most) and the terminal's stdout aren't. My LANG env variable is set to en_GB.UTF-8, so what am I missing?

Just look at the man pages for less and more. Ignore the command options and concentrate on variables. Things will default to the en_US C or POSIX.

What about $TERM $LC_CTYPE $LC_ALL and all the others less looks at?
What about /etc/environment? Nobody can steer you through this without an immense amount of time. If you want it, go after it, and keep backups.

BTW, an unwritten rule in these things is that when someone offers to help and asks a question, you answer it as precisely as you can. Otherwise, we end up talking with you when you wander in circles, and nothing gets done. Let your next post include the answers to what has been asked.