so would not simulate live to the ground of the garage ????
is there another type of test you can do to simulate this.

What do you mean by ground of the garage?

Is there any structural metalwork, or metal services, such as water
supplies? If so, they should be main bonded back to the house, which will be
very difficult being 40m away.

Your garage might be better rewired as a TT system. Isolate the earth at the
garage end and install an earth rod. Keep the RCD and ensure the incoming
cable goes straight in and the consumer unit is a plastic insulating type,
not metalclad. Check the earth loop impedence is nice and low.

Christian.

P.S. you can test the RCD against any earth by getting a 13A socket with
flying leads. Connect the live to the circuit you wish to test. Connect the
earth to the earthing point you wish to test. Plug your RCD tester into the
socket. Perform test.