I want to receive freeview on the pc (has a dvb tv card) in the back room, but only the rooftop aerial seems to work, which is hooked to a wall plate in the front room. Are there any extension cables (i need around 40m) to connect them?

You can easily make up an extension, cables and connectors are very available on the internet.

If this is a long term exercise then you would be better installing a splitter/distribution amp in the loft (assuming the old one goes that route) and wiring a new socket in the back room. If you can't do this yourself there are plenty of tradesmen who can.