To test the earth rod with an earth loop impedance meter you are also testing the boards earth. The board is only required to have a reading of 21 ohms outside the installation so it is possible even with a perfect earth using an earth loop impedance tester the reading would still be 21 ohms.
Only with the two test probes of an earth rod test rig can one really be sure of low readings being really what the rod is giving. Even then to get 8 ohms I was using between 4 and 8 rods at a gas receiving station where I sunk in 100's of rods.
With all water pipes etc disconnected 13 ohms would be a great reading and if I was presented with those readings I would be uneasy as likely the water pipes etc have not been disconnected to get such a low reading.
I would expect around 60 ohms in a domestic which is low enough to trip the 100ma RCD and likely still stable even in dry weather.
If you want a reading below 30 ohms then don't use an earth loop impedance meter use the special earth rod test meter.
I admit many look at the reading on the earth loop impedance meter with all bonding still connected and consider that it does not really matter if the earth is from the electrode, water, gas or any other metal work as long as it is there. In some ways that is correct. However we can't rely on water or gas and one month after testing the utilities may change the material the pipes are made of. So I always test the rod alone when doing an inspection report. However when doing other work then I will take the readings presented as correct if they seem to be what would be expected. Marked of course as "by enquiry".
But only by measuring on regular basis does anyone know what to expect and I would think from comments bary80 does not know what to expect. I have installed many rods, but I don't remember installing or testing a single rod during my apprenticeship, or in college, we only did theory, so I can understand why an electrician would have problems with this.