RewriteEngine On
RewriteBase /radio/visualising_radio/mobile_message/

# so say your query string is ?cat=hello
# this should make the variable %1 equal to hello
RewriteCond %{QUERY_STRING} ^cat=(.*)$
# this should take the person to http://www.bbc.co.uk/hello/
RewriteRule .* http://www.bbc.co.uk/%1/