I know that due to the way the hot and cold water come through the
bathroom
wall, they are pretty much touching at one point - could this be the
problem? Or is it more likely something to do with air in the pipework?
Don't know if this makes any difference, but at the moment, the branches
for
the basin and toilet cistern are not connected to anything.

I had this problem with a pipe that went no-where. I think when you get the
air out it will go away. Air is compressable - water isn't !

Paul.