Cheers TPC,
Ok so my antenna is just a real cheap cr@ppy one so the first thing ill try is a new, better quality one. I think i will try to keep it on the bullbar as theres always a lot of trees where we drive and it would probably get a caining on top of the vehicle.
So what is the 1/4 wave whip? Ive seen interchangeable whips (ie: 6.5db antenna with a 3 db interchangeable whip) but havent come across the term 1/4 whip. Does it just reduce the 6.5 to a quarter?
Thanks for the link, i have seen that diagram before and so decided that 3db is the best. It will be very rare for me to need long distance and even more rare to have a long line of sight to the next vehicle. 90% of its use is less 500 metres BUT through very densly forested and hilly terrain.
I have started looking at the GME range but theres so many varients! So far i think i need:

3db for the terrain im in (possibly with a 4.5 interchangeable whip).
Around 600-800mm tall to help help clear the roof line from the bullbar mount.
Stainless steel as opposed to fibreglass to help with tree strikes. (perhaps with one of those lock pins in the spring).
A ground plain independant.
Nice thick heavy duty/good quality coaxial cable.
And my fingers crossed that i havent wrecked my UHF unit!
Does that sound like im on the right track?