Need to get a long ethernet cable, or maybe homeplugs?

[nev24]

I would recommend you use home-plugs. If your media pc is going to be far away people could trip on it etc, with a home-plug it get's rid of that issue. Or you could get an wifi dongle.

[badass]

Well I've seen on ebuyer cat6 30m cables for £7, seems to have good reviews, and they're 'branded' with a limited lifetime warranty, so I think I'll just go for it. The 30m means I should be able to route it so it won't show much which will hopefully keep everybody happy.
I'm assuming I may as well go for cat6 over 5e since it's only £1 more expensive?

I'd go for it. As said below, CAT6 is not as flexible but I haven't had any problems wiring my house with the structural (the stuff that connects sockets) CAT6 cable. The bend radius is fine unless you need to route it around really tight corners.
Also, contrary to popular belief, Cat6 cable IS rated to 10Gbit. Just only to 40/55 meters.

[peterb]

You might find this easier to route under carpet. Not used it myself, so I can't offer any opinions on the quality.

http://www.amazon.co.uk/98FT-Etherne...+network+cable

There are other lengths available from suppliers on Amazon market place. Just search using flat network cable

This for example

http://www.amazon.co.uk/20m-Flat-Net...98JP72N3HG790Z