A HDMI splitter would give you a second HDMI output, but you would then need a converter to change the second HDMI output into an analogue input, your DMR EX88 accepts RF input, S-Video, and AV Video.
You can buy some cables which claim you can go from HDMI to AV without a converter, however based on my experience this doesnt work, as the process of converting from digital to analogue is rather complex, so I would pay the extra £10 and buy a converter rather than risk buying a cable which doesnt work.
If you purchased a converter such as the one here, this would take the HDMI input from your SKy box, then convert it to a S-Video or Video out, which you DMR EX88 supports as inputs, if you went down this route you would need the converter, an HDMI cable to take from the splitter output to the convert, then a S-Video or AV cable to take from the convert to the DMR EX88.
Another option would be to use an RF modulator such as the one here (which is what the old sky plus boxes have inside), and that would take a HDMI output from the sky box and then turn it into an RF channel which you can pick up tuning your DMR EX88 into a channel (like you do today). In terms of cables required you would need a HDMI cable from the splitter to the converter, then a coax cables from the convert to the DMR EX88.
The challenge in using either conversion methods is two fold, firstly alot of Sky channels are copyright protected so you may find some things are not recorded this is due to the converters not supporting HDCP, secondly your changing from digital to analogue and as such the video quality may be rather poor.
If you have any follow up questions please let me know