Hola.
Gracias jantoni. He hecho lo que me has dicho pero no hace caso sigue saliendo la plantilla y el index.html igual y de hecho en skin.conf me aparecen las unidades en US:
 [[Groups]]
        # For each group of measurements, this section sets what units to
        # use for it.
        # NB: The unit is always in the singular. I.e., 'mile_per_hour',
        # NOT 'miles_per_hour'
        group_altitude     = foot                 # Options are 'foot' or 'meter'
        group_degree_day   = degree_F_day         # Options are 'degree_F_day' or 'degree_C_day'
        group_direction    = degree_compass
        group_moisture     = centibar
        group_percent      = percent
        group_pressure     = inHg                 # Options are 'inHg', 'mmHg', 'mbar', or 'hPa'
        group_radiation    = watt_per_meter_squared
        group_rain         = inch                 # Options are 'inch', 'cm', or 'mm'
        group_rainrate     = inch_per_hour        # Options are 'inch_per_hour', 'cm_per_hour', or 'mm_per_hour'
        group_speed        = km_per_hour        # Options are 'mile_per_hour', 'km_per_hour', 'knot', or 'meter_per_second'
        group_speed2       = km_per_hour2       # Options are 'mile_per_hour2', 'km_per_hour2', 'knot2', or 'meter_per_second2'
        group_temperature  = degree_F             # Options are 'degree_F' or 'degree_C'
Es como si no hiciera caso de skin.conf.
Entonces he deshecho éste cambio y he ido a modificar el archivo weewx.conf, en el apartado StdReport, y allí las unidades son exactamente las que me salen en las plantillas (meteoclimatic.htm e index.html) pero si modifico aquí meter_per_hour por km_per_hour entonces no me genera las citadas plantillas (meteoclimatic.htm e index.html), hay algo que le provoca confusión:
[StdReport]
    # Where the skins reside, relative to WEEWX_ROOT
    SKIN_ROOT = /etc/weewx/skins
    # Where the generated reports should go, relative to WEEWX_ROOT
    HTML_ROOT = /var/www/weewx
    # The database binding indicates which data should be used in reports.
    data_binding = wx_binding
    # Each of the following subsections defines a report that will be run.
    [[StandardReport]]
        # See the customizing guide to change the units, plot types and line
        # colors, modify the fonts, display additional sensor data, and other
        # customizations. Many of those changes can be made here by overriding
        # parameters, or by modifying templates within the skin itself.
        # The StandardReport uses the 'Standard' skin, which contains the
        # images, templates and plots for the report.
        skin = Standard
        [[[Units]]]
            [[[[Groups]]]]
                group_altitude = meter
                group_speed2 = meter_per_second2
                group_pressure = mbar
                group_rain = mm
                group_rainrate = mm_per_hour
                group_temperature = degree_C
                group_degree_day = degree_C_day
                group_speed = meter_per_second
Por tanto, si lo modifico en skin.conf no me hace caso, y si lo modifico en weewx.conf no me genera las plantillas.
Sigo investigando...