The Sky box already has everything inside to do this, you simply need a "Magic Eye", these are freely available in UK TV shops, or direct from Sky. You MUST connect it to the RF2 output, and turn the power on in the installation menu, this sends 9V up the coax to feed the magic eye, along with the UHF TV signal. The magic eye sends an RF carrier back down the coax to control the box.
The picture menu of the box also has seperate aspect ratio settings for RF1 and RF2, so you can set them to suit your TV's - the output is the same for both, but it will switch according to which remote you use.
It's ABSOLUTELY vital that all coax connections from RF2 are crimped or soldered, otherwise you get problems intermittent power to the magic eye - and if it goes through splitters or amplifiers, these MUST be specifically designed for use with a magic eye.