"Mark" wrote in message
...
Some time ago I fitted a new socket outlet in my son's bedroom. I had
no desire to go into the wall and hook into the existing ring main so
I decided to tap in somewhere else. Basically I tapped into the
immersion heater supply in the airing cupboard, (fused 30amp at the
consumer unit).

What size cable is this circuit using - 4mm or more I hope, if it is 2.5mm
you need to replace the 30A MCB with a 20A one, unless the immersion is on
it's own 2.5mm ring.

If you have an immersion, I hope this is connected with an FCU, fused at 13A
(Unless you have the unlikely scenario of an immersion heater greater than
3KW)

Then I went through a 13amp fused spur, then up into
the attic, and then down into the bedroom using mini trunking, and
finally connected an RCD protected double socket. Everything fine.

Possibly fine, if the immersion heater cable is 4mm (or more) or on a 2.5mm
ring.

Now I need to do the same in my daughter's room. My question is, if I
wire in parallel from the feed to the socket in my son's room do I
need to use another RCD protected socket? Or will the RCD in my son's
room provide protection for a regular parallel socket.

Unless the socket in your son's room has a dedicated RCD protected output,
and you use this, you need another RCD socket, or just replace the fused
spur with one of these.
http://www.tlc-direct.co.uk/Products/CM4904.html (Then any further additions
will be protected)

If you just connect a normal socket to the same terminals the existing
socket is being supplied with, it will not be protected.

Safety is paramount of course but I don't want to spend 20 odd quid on
another RCD socket if I don't need to.

Incidentally the sockets are only for hi-fi/TV so I doubt the combined
load will ever exceed 5 amps.

This is irrelevant, you are providing a 13A socket, it needs to be able to
support this load - someone may plug in a heater at some point.

Sparks...
(Not an electrician)