At 534 ohms, the Ra is well within 7671 requirements, but is the value stable and reliable? Might there be a reason why it is relatively high, is it in to loose soil, poorly connected etc?
The Ra at my own house was around 300 ohms when last tested but I have made several tests over the years and was able to confirm that it was a consistent result. 
I live in the foothills of the Mourne mountains with rock bursting through parts of the garden, not so good for sinking rods. 

At 534 ohms, the Ra is well within 7671 requirements, but is the value stable and reliable? Might there be a reason why it is relatively high, is it in to loose soil, poorly connected etc?
The Ra at my own house was around 300 ohms when last tested but I have made several tests over the years and was able to confirm that it was a consistent result. 
I live in the foothills of the Mourne mountains with rock bursting through parts of the garden, not so good for sinking rods. 

The ground is well compacted. The rod is close to the house wall driven down the side of the concrete yard. The houses are well over 100 years old and have no footings and also the ground is about 2 ft lower than the garden to the rear which is chippings so free to drain. There is no chance it could freeze or dry out under the concrete, except in exceptionally severe weather.  The area is pretty flat and below the soil there is a lot of sand. I have three other properties within 100M and they about 100 ohms or lower. I haven't checked yet, but I'd be more inclined to suspect the termination on the rod.