The problem mentioned by Xplicit above did use to be an issue if the 2 devices were on different rings (or radials), however I do believe that was addressed some time ago.
They can be secured, and Big Al is correct when he says that the properties either side should be on a seperate phase - but that doesn't mean they are.
From looking at the specs, 85Mbps is the absolute maximum bandwidth. However by the time you invoke the security and take in to account how dirty the mains could potentially be, along with the length of the cables themselves - you're probably looking at getting half that.
If it were me, I'd be running Cat6 and hardwiring.
On the plus side, Maplin do have a fairly good returns system in that providing you keep the packaging, etc then you can return whatever you buy within 14 days for a refund and it won't be a problem.
The only thing you'll "lose" if you buy them and they don't work for you is your time in buying and returning them.