You can get a hdmi to dvb-t modulator which will allow you to distribute the HDMI over your existing RG6 (Coax) cable at a much higher quality than your existing modulator will be providing. However these aren't cheap. If you're easily able to, I recommend just running an HDMI cable.
If you were previously using the RF mod to feed another room, When Sky replaced your decoder the should have given you another modulator.
They did when I got my decoder upgrade a months ago.
They would probably even send you one if you rang and asked.
You could run the Sky Video out via the RF cable but the snag is you would need another cable for Audio. I'm doing this to slave a small screen to the kitchen.
Thanks for your reply. Sky actually came around and sorted it for me.