With the new code then once the average is above ~4-5mph the Vortex is reporting averages slightly above the OS anemometers, and gusts of up to around 25-30% higher. This seems consistent with relative positioning and the expected ‘advantage’ of 3 second gust measurement over 14 second gust measurement (I recall reading a topic where someone did a empirical analysis comparing gust speeds from various time spans).
By observation the Vortex cups do start in speeds as low as ~1mph (i.e. >2 seconds per rotation!), but nonetheless it seems that when the 5 minute average is below about 4-5mph, the OS anemometers (especially the WMR200) record more wind than the Vortex. It seems possible that this is due to ‘local factors’, e.g. what little air movement there is having to go ‘around’ the house and that effect being spotted by the lower anemometers? Intruiguing, but hey, it’s probably really the windier days that I’m more interested in. 
Two graphs attached, one showing the max gust in each 5 minutes for the last 24 hours for all 3 anemometers, the other showing the 5 minute averages for the same period.
