Could be the cable is a low quality one and cant do 1080, but is fine with the low res DVD player, or else the DVD player just has a better output than the sky box when dealing with an overspec cable length.
HDMI (as in the organisation) has recently moved away from "versions" to simply HDMI Standard speed and HDMI High Speed.....
All I am saying is that if it's a new cable (say in the last 12-18 months) and it states a version, it's possibly just a chinese knockoff cable and may or may not comply. 5m is actually a decent distance to send a 720p (I think Sky is 720p) signal.... the DVD player may not put out that high a resolution hence it may work.
The bigger issue could be an EDID problem as this is currently the problem with longer HDMI cords (tho typically 10m long) rather than the data stream