that's just what you want to make the garage a pukka TT system. If you
did connect the house earth to the garage rod, under fault conditions
(not necessarily in your own house, but for example another place close
by on the same phase of the same substation) you might end up with quite
substantial fault currents coursing through your earth wiring; not really
desirable...

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly TT,
thus potentally unsafe as you describe!
(This is why I asked :-) )

Can any one shed any light on this question!?

(Another reason I ask this, is, if a generator is installed, this has to
have a rod (AIUI), but if this has a rod, connected to the house earth
supply, does this them break the earthing rules on my TN-S system?)

Transfer switches only seem to switch the L&N (Via an "all off" position)

Sparks...