The OP is asking for a rule book for connecting outbuildings but there isn't one; it depends.
Agreed.
My point is that in a house with TT, the outbuilding could be nearer the electrode and MET than other exterior parts of the house yet little consideration is given to this.
It could, but I'm not sure what sort of 'consideration' you feel should be give. No matter how near or far the outbuilding (or the house) is from the TT electrode, the potential difference (between house and outbuilding) I was talking about was the possibility of, say, an 'unbondable floor' in the outhouse which offered a path to earth, whereas the house should be constituted as an equipotential zone.
[In case someone quibbles, I would agree that an exposed-c-p within a house which can be touched by someone standing outside the house (e.g., again, a metal light switch just inside the door) presents the same potential risk - so, for the risk-averse, probably should be avoided.]
Should outside taps at the far end of a house have an electrode?
No. Taps are not 'electrical' (i.e. not exposed-c-ps) and therefore do not need to be earthed. As I've said (and bernard has said more often!), the ideal would be for them to be electrically isolated from the house's earthing system (e.g. by plastic plumbing).
What is the difference between them and an outbuilding not far away?
Nothing,
IF, like the tap, the outbuilding has no exposed-c-ps, associated with electrical items.
If there are extraneous-conductive-parts then, effectively, there is (may be) already an electrode in place (I know it may not be allowed as such but it is) and if this extraneous-c-p is a metal water pipe from the house there is an electrode which is connected to the TN earth so why is connecting a true electrode to the MET so undesirable?
IMO, it clearly is not 'undesirable' or, at least, even if some people regard it as 'undesirable', it is inevitable. As you say, there is absolutely no difference between something called a TT electrode and a metal service pipe entering the property. If it is not only permitted, but very much required, that the latter be connected to the MET (in any installation, not just TN), then it is totally irrational to suggest that connecting a "TT electrode" to the MET is 'not allowed'. As we've been told, in a good few countries (inclkuding the US, IIRC) a local TT electrode is actually
required with TN installations.
Kind Regards, John