Just to round this off, in case anyone is interested. 

The solution I wound up with is based on this kit from Jaycar.

https://www.jaycar.co.nz/bidirectional-ir-extender-over-cat5e-100m/p/AR1809

This is meant to be used with one of the two identical boxes at the TV end, with an IR receptor on a rattail. The other box goes at the SKY STB end, with IR emitters on a rattail. The rattails connect to the boxes with 3.5mm phono-type connections. The two boxes are intended to be connected with Cat5e cable. Only one of the boxes needs to be powered - a 12v wall adapter is included.

However, as mentioned, the two boxes are identical. Either box can be TX or RX and each has IR-in and IR-out. This meant that I could use just one powered box with both IR-in and IR-out, all in the vicinity of the Sky box. The IR receptor is outside the Sky cupboard and everything else is inside - no Cat5 cable involved. Works well.

This means that I have a spare box, and spare IR-in receptor and IR-out emitter rattails - enough for another whole set-up like mine (apart from the power adapter, which could easily be sourced).

I'm open to selling the 'second kit' if anyone is interested. Manual is here:

https://www.jaycar.co.nz/medias/sys_master/images/9174633873438/AR1809-manualMain.pdf