I think you are referring to the 800 duck antenna sold by RS. It works fairly well, but if you really want a GOOD 800 antenna, get the one made by Antennex sold at Scanner master. (www.scannermaster.com). It is the best I know of.
I find my home-brew 1/4 wave ground plane, hung-from-ceiling, is more effective than the "800 MHz duck". This is no surprise yet the 800 MHz duck is better than stock for 866MHz APCO 25 than the stock antenna provided.
I believe the stock antenna is bettter than either for VHF though.
Your antenna depends on your main listening band. Here I go with different antennas for differenent bands on the Pro-96...
depending on your listening bands do you have mostly 800 mhz freqs on your scanner or a mixture of all ranges from 20 to 800 mhz if so then a 800 mhz attennas will help with just those 800 freqs youll be able to recive lower freqs but not as good as the stock ducky attenna on my rig i have both 800 mhz and the stock attenna mounted on my roof it works well