So how do you feel about running in a suitably sized separate CPC that can take care of both the bonding and circuit protective conductor in the event of your DNO supply being PME'ed?? In the meantime it can be used as a link to an additional earth rod position at the garage via the garage EMT back to the MET in the house, and providing a suitable means of bonding to the extraneous water pipe in the garage. The TT rods can be left connected to the MET/EMT if and when a PME connection has been made available....
As davesparks has suggested, the existing Ra can be significantly improved from 14 ohms with a little thought and effort. Plus the lower the Ra achieved, will make all the difference if and when a PME earth is provided by the DNO, not to mention the existing TT system....