dotto,
Only the bathroom rad needs cross bonding.
Breezer is right. If you can fix to the studs that's the way to go. You may be able to just screw a stud or strap across two studs and use the stud or strap as your mounting anchor. However if you find the studs are too far apart to do this or it wouldn't look right then the job is going to be a little bit more involved and will require a bit of plastering.
The method:
You may well find that you want or need to mount the rad between two vertical studs or indeed between three (i.e. one in the middle of where your rad will be mounted). You're going to have to 'chop' into the stud wall and add a 'stretcher'. To do this, you'll need some 3x2 stud (available at your B&Q). You'll need enough to add 2 stretchers. One for the bottom screws of the rad bracket and one for the top screws. Mark where your rad will be mounted. Then you'll need to locate where the vertical wall studs are running. Once you know this and you know where your mounting holes need to be, you can chop into the wall (only about a 4inch gap horizontally between the wall studs. Once you have a channel chopped out, insert a length of 3x2 just long enough to span between the wall studs and screw this new horizontal stretcher into the vertical studs at both ends. Use decent size screws and pre-drill the holes in order to get a good fixing. Once your stretchers are in place you'll need to cover them by plastering over using plaster board and then plastering up. Or if there are no big gaps just use PVA to pre-treat the area to be plastered then plaster straight on top. Once its all dry you can mount your brackets and mount your rads!
The same principle can be used and plastering can wait, if you chop into the 'other side' of the stud wall. i.e. chop into the wall in the room that the stud wall divides. Thus enabling you to get on with mounting the rads and as i say, plastering can be done later assuming the room isn't used.
Hope that makes sence.
