In reply to Indy:
> I'd like to be able to calibrate some equipment
I'm not sure how accurate you want to get, but…
Horizontally, most late-model consumer outdoor handheld GPS units do the averaging if you leave them on for a while and should be accurate to within a couple of metres. i.e. probably get 5m accurately straight away, 3m if you leave it on the spot a few minutes, possibly less than 1m if you sit there half an hour.
Vertically, most of these are pretty rubbish and are regularly out 20m-30m vertically, despite what it says on the box. Leaving it turned on there will help a bit but I wouldn't expect better than 5m accuracy vertically, if that.
If you want sub-metre accuracy in the vertical plane i.e. exact altitude, you need a DGPS and process the results.
I'm sure some of the OS trig points are very accurate. Some not so much.
Google Earth is amazing but the heights can be WAY out, depending where you're looking, the quality of the imagery etc.
SRTM (space shuttle) data is good, but has gaps and other vagaries similar to Google Earth and may not be in a form useable to you.
Post edited at 12:15