Actually I don't think that the Maplin adaptor will be a suitable prototype/cheap solution as it is not a true balanced/unbalanced converter so its still unbalanced over 3 core. May or may not be a problem over the short distance needed if unit placed behind processor. I don't know as there seems to be
different ways of doing AES<->SPDIF of which AES->SPDIF is simplest but its all down to what voltages are used. Some of the
Neutrik adaptors may be better as they are transformer based and still relatively inexpensive at about �40 giving a total solution at under �50 per 2 channel.
Alternatively it may be relatively easy to convert balanced/unbalanced using a component video balun such as
this which if sympathetically wired, may do the RJ45/phono all in one and just need a suitably wired CAT6 cable - you would lose the 3 unused connections, effectively using only one of the three component video channels to transmit the signal. Worth a call to madnat to see how it is wired internally - they make some of their own kit so may be possible to get a custom job.