sky hd box has optical audio out use a toslink from there to the aiwa, it would only mean that the sky box would be inputting surround.
have you checked the tv for an optical out?
I have an onkyo amp which has several optical ins and outs plus several HDMI ins/outs, no scarts if I remember, it was good value around £300. Unfortunately it may be a case of having to upgrade various bits.
there is a setting in the sky box for what audio out you use, digital or otherwise, check the settings in there as well.