There is certainly no prohibition on RCDs with identical trip thresholds feeding one another - indeed if you are supplying a caravan or a canal boat it is de-rigeur. In your shoes I'd only do it if there is someone there to reset it without too much aggro - if the first RCD is 100m away in a locked cupboard I may be a bit concerned about the ease of resettting versus the rising chances of a trip at an awkward moment. An all RCBO board solves this but is over the top for adding the odd  garden skt.

~The exploding hot tub sounds fun though and perhaps far more dangerous than the electrics.
Mike.

I had to re-read the final part to understand the last part of your sentence ;-) 

So TT’ing a single socket is fine (whilst of course ensuring all other PME’d stuff is >2m away).

RCDs of the same spec in series is always talked about as a big no no, but I see no issue with it. 

Thanks. 

vantech said:

TT island

If the installation is connected to the DNO supply, how is it an 'island' ?

TT installations, with their own earthing arrangement, may operate in island mode, or connected mode, just as TN installations may operate in connected mode or island mode. Similarly, parts of installations that have their own TT earthing arrangement may operate in connected mode or island mode?

Furthermore, anything connected to the general mass of Earth is connected to everything else connected to the general mass of Earth.

Sorry wrong use of the word “island”; so the “parts” of an installation in this instance would be the socket, connected to earth by means of a rod rather than the suppliers connection. 

assuming the cable to this final socket is armoured, the gland and the armour would presumably be connected to the PME earth. I read somewhere that the shroud which usually covers it might not be suitable for ensuring it is not an "exposed" conductive part, so might need further thought

Thanks, I would likely land the armoured cable into a separate enclosure and then take a 2 core from this enclosure into the socket using plastic conduit or a 2 core outdoor cable of some description. 

why not have the gland inside its own enclosure directly below the socket? That is if i am right in my thinking. So "exposed" becomes "enclosed".

No worries then about two core cables trailing around without any earth in them, in case they were damaged or cut

vantech said:

I usually TT distribution boards as a whole

What! All of them?

I think vantech is referring to installing a supplementary earth rod to each DB. Which of course doesn't make it TT.

BTW Chris, I heard a podcast recently about black holes and dark energy here share.fireside.fm/.../2YAGasSP QG0ltBCi seems to be from you, are you a dark horse?

A "tt island" within a PME supply is a vernacular term used to refer to somewhere you take the live and neutral (or even PEN) from a TNC-s supply, but do not use the neutral derived earth - rather using a local electrode. Such is common for EV chargers, and outbuildings on farms, where the DNO provided earth may be either used only for the domestic bits of the farmhouse, or more commonly round here, ignored altogether. The neighbours may use PME however, so it tends to join up a bit at the water mains etc.

Furthermore, anything connected to the general mass of Earth is connected to everything else connected to the general mass of Earth.

Indeed, but often only quite loosely, and where a voltage difference is impressed between two earthed objects from a low impedance supply, most of the voltage drop occurs within the near field around the point of contact, typically more than 60% within an excess radius  around the electrode (or item serving as an electrode) comparable to its principle defining dimension(s).
Hence concerns about fences around the bases of poles to avoid surface step voltages affecting livestock, especially with longer body dimensions - cattle are easier to electrocute than sheep or dogs,  and in turn both of those are more vulnerable than chickens !!

M.