It is recomended on PME earthing systems to use a seperate earthing arrangement in the out building, as this doesnt normaly form part of the equipotential zone of the house.
However, this is a recomendation from the REC, and is not a regulation from the IEE.
If you were to apply this rule in general, all outside lights, sockets etc would need rodding down.
You should also be aware that any rod installed SHOULD be tested to ensure the impedance is of an acceptable value, a job to be done by a spark, as he will have the correct test kit.
It is possible to carry the equipotential zone of the house to the garage, and this must be done in any case (and the same earthing system used from house to garage) if there is any common metalwork such as water, gas or central heating pipework.
If you do use a seperate earthing arrangement, you must ensure that the cable from the house, if armoured, has the armour earthed from the HOUSE side, and this must be fully insulated from the garage side.