That will be a tn-s system and is the suppliers earth to met, so providing you have a ze lower than 0.8 ohms and the consumers earth from the met to the consumer unit is of an adequate size its of no concern to your pir. If on the other hand the small wire is coming straight off the sheath into the consumer unit i usually reterminate into an external met and take a 16mm into the cu it will then satisfy the cert as the consumers main earthing conductor is of the right size. you should neva mess with there incoming cable, if you take a ze reading and it doesnt satisfy you contact the supplier to upgrade hope this may help.