Do you have a reading for Zs at the distribution board Then simply calculate an approximate PFC as V/Zs if the value is significantly lower than the suspected 3ka breaking capacity then no problem. 434.1 PFC calculated measured or by enquiry.
What size are the BS88 Fuses on the supply side...
If the garage is protected by the RCD in the house why not replace the Garage RCD with a double pole isolator as used in older CU's as a main switch
You then have a spare RCD and discrimination is not an issue.
