Experience shows that with a good view of the sky, and therefore a good spread of satellites, reproducibility can be better than 5 metres and with EGNOS perhaps half that. EGNOS corrections are broadcast from an equatorial geostationary satellite which isn't visible if you've got much of a hill to your south. Like Graham I'm not convinced by altitude figures. I have read that the vertical inaccuracy is about 50% more than the horizontal, but actual results are worse than that ie about double.
One issue when creating databases is what to store and publish for coordinates. WGS84 is a given (or ETRS89 if you want to future-proof against continental drift!), but we should supply NGR as well. There are two possible ways to convert WGS84 to NGR. To match the actual grid used by the OS which has built-in distortions because it was surveyed by hand over 60 years ago, you need to use their on-line converter. The alternative is to use the mathematically precise and much simpler converter built into your handheld GPS receiver. The first will match published OS maps, the latter will lead another GPS user more closely to the right spot on the ground. In Assynt the difference is about ten metres. I think the latter is the better choice.
I'd expect all users to have their GPS receivers set to NGR in order to use OS maps, so we do need to supply NRG as well as WGS84 and it should be a NGR that leads the user to the right spot and not one some distance away. Not everyone will do as Jarvist recommends and use WGS84 all the time.
Some numerical results:-
66 measurements taken over several years on top of a pillar in front of our club hut gave a range of 10m and standard deviation of 2.1m for E/W and a range of 20m and standard deviation of 3.3m for N/S. The range for altitude was 39m with a standard deviation of 6.0m. All readings were taken after selective availability was turned off. I used a Garmin GPS12 at first and later a GPSmap 60CSx. EGNOS was not available.
At the nearby OS passive station the ranges and standard deviations for 13 measurements again over several years were 8, 8, 17m and 1.9, 2.8, 5.8m for E/W, N/S and altitude.