To check on the right coordinates of OS active stations, see the link below 
http://gps.ordnancesurvey.co.uk/active.asp

Conversions:
http://gps.ordnancesurvey.co.uk/etrs89geo_natgrid.asp

Base station data not 1 second resolution?  Resample (interpolate using posgps).  While this doesn't improve the data, it apparently makes it more evenly applied (no internal interpolation?).  Unverified.