Thanks for the last two very useful replies...

The desired location for the bike aerial is lower part stuck to the
inside of the screen with as much as high and above the screen as
possible.
The reasons for this are obviously its good to have it as high as
reasonably possible, but also to get it away from the QRM generated
by the electronics around the dash area.

Also, as we ride as a pair, one is always in front or behind the
other, so immediately there is the body mass in the way, screening
the signal. Now this doesn't generally matter as we are usually
withing 200 metres of each other and 1/2 watt at UHF is still full
strength signal, but when we get perhaps a few miles apart and are
in front / behind situation, the body is definitely attenuating the
signal significantly.

Generally one rides in front much of the time, so a rear aerial
gets round this, and a front aerial on the following bike is not
shielded by the riders body - I have both front and rear aerials
and I guess it would be good to have an antenna switch to select,
but BNCs and swopping is no trouble.

The other thing is that everything has to be waterproof where it
matters. I have been experimenting with hot melt glue to pot up
connections - anyone have experience of this as a good potting
agent ? I will put out a separate thread about this as its a bit
hidden in this thread..

Thanks again all - much appreciated

Nick