You're quite correct. I was thinking the red was also connected to 1; which leaves two spare wires - the red (3) and black (not connected). I suspect there are two separate cables: red and black (old style); and (new style) brown, blue, green/yellow and one with white sleeve conneected to 3. If that is correct, the red/black cable is redundant and can be disconnected (easy to check) and made safe, together with blue and green/yellow. That just leaves brown and the wire with white sleeve as live and switched live, which go to A and B, respectivly, on the DT90E.