I agree, if you can manage it, it would be best to expand the ring to include all the new sockets. This is assuming the conservatory is a definite permanent and waterproof addition to the house. If there is any doubt about this it might be better to stay with just one spur coming in.
The first FCU must be BEFORE the existing spur socket in the existing room. Everything spurred must go after this first 13A fuse. You could put in the FCU either next to the existing socket (make a hole in the wall for the box on whichever side the cable enters and then go on with new cable), or next to the socket where the spur cable starts (if it starts somewhere else in the room at a different socket)
The second FCU would go where it is most convenient.
The first FCU must be BEFORE the existing spur socket in the existing room. Everything spurred must go after this first 13A fuse. You could put in the FCU either next to the existing socket (make a hole in the wall for the box on whichever side the cable enters and then go on with new cable), or next to the socket where the spur cable starts (if it starts somewhere else in the room at a different socket)
The second FCU would go where it is most convenient.