Benwood
Noo ........ don't trim out the radials - at least not yet.
I am not familiar with the Scantenna antenna as a design in its self, but the general generic relationship between antenna element length and frequency (at least with broad band omni-directional type antennas), suggests that lengthening elements is the way to go if improvements in the lower end of the overall antenna coverage is been sort, and that shortening is the way to go if improvements are sort in the upper half of the antenna band width.
I would be starting off with an attempt to establish which part of the Scantenna is responsible for VHF reception e.g. in Discone design it is primarily the length of the downward/outward slanted radials that account for VHF performance. With that principal in mind (i.e. the relationship between frequency and antenna element length) do some research online, or make a few phonecalls to a Scantenna manufacturer to try and establish which part (which radials) on the Scantenna account for VHF reception.
You'll get a lecture about how changing the length of any radial is going to affect overall performance, isn't good for the antenna, blah blah, blah blah ....... but, if thats what you wish to do, and you are comfortable sacrificing performance in the UHF band for improvement in the VHF range, then it should be achievable to some degree.
If you just "want to go for it" so to speak, speaking for myself, I'd start off by lengthing the appropriate radials, say by about 30%-50%.
See now how performance is affected (and make sure you make your comparison/s with the receiver tuned to the same frequency/signal and modulation method used both before & after), and see how UHF reception is impacted (is UHF now sounding any worse or better?).
There will certainly be an effect on antenna resonance, as well as a shift in the antenna design center frequency - at least a measurable change to these characteristics if the experiment is carried out in a chamber, but as I have so often said in my comments: measurable changes in any antenna performance do not neccasserily translate into real world performance/listening changes - which I guess is what is more important for you.
An any event, if there is a VHF reception/listening improvement with the relivant radials extended, then you may wish to try and optimise the improvement by experimenting with different increases in radial length. However, if there is a drop in performance, well then, shortening may well be the way to go, but the chances are that lengthening is going to be the right way to go here (with the right radials lengthened, that is), and last but not least: it's a darn side easier to start off with lengthened radials, and to slowly trim them, then it is to start off with shortened radials, and to keep lengthening them!
Good luck