I've discovered that there is a parameter used by Windows Time service called "Last Clock Rate" (along with MinClockRate and MaxClockRate). Apparently this figure is used as a counter by the RTC. When the counter reaches zero the RTC produces a "tick" (I believe there should be 100 ticks per second).

The accuracy of the RTC is maintained by adjusting this value. I've found that on my pc this value is set to the default, which should almost always not be the case - no RTC clock is that accurate. I suspect there is probably some other service which allows the RTC to be altered, but which I have disabled (probably as advised for "security" purposes) .