Looking for some advice on what is required for earthing arrangements on a detached garage. I am in two minds about the need to convert the garage to a TT island.

House supply is TNS, but I probably have to assume it may be converted to TNCS in the future.

The garage is about 1.5m from the house, it has 2 double sockets and an LED light installed.

Feed to the garage is via steel conduit running down the house, through a concrete path and then up and along the outside of the garage. Then through the wall to a steel junction box and then plastic conduit to metal clad fittings.

If earth connection is disconnected from the steel conduit, testing with a continuity tester shows it to be isolated from earth, haven't tried an insulation test so far. There are no other services entering the garage and it's brick built without a steel frame.

At the moment I have the steel conduit connected to the cpc of a 2.5mm T+E cable and then single core 2.5mm earth cable continuing the earth from the steel junction box to the garage installation.

If an EV was connected to a garage socket for charging in the future it would have to be via a 15m cable. I think it would be far more likely that a EV charger would be put on the outside of the house close to where the car will be parked.

So can I say that as :-

The steel conduit is isolated from earth and therefore not an extraneous conductive part, no other extraneous conductive parts.

Sockets unlikely to be used for EV charging

Therefore OK to use TNS earth from the house without bringing 10mm earth bonding cable from the house???

Alternatively 

I could isolate the garage and put a an earth rod in

I could probably get a 10mm earth bonding to the conduit at the house side but getting the cable to the garage would require replacing the conduit which will be quite expensive for the customer on a job that's gone way over budget already due to a multitude of problems hidden under loft insulation and inside walls.