Brilliant - thanks. I am confident in the 400mm centres as I saw it being built; I just didn't think ahead to take a measurement to the first one. I'll use a small hole to locate as you suggest.
With the cellotex I'll therefore just make sure I've got long sturdy screws to go far enough into...
I have an external wall (dormer) which is plasterboard on top of 25mm cellotex over the normal timber studwork.
I want to fasten shelf brackets to the wall. If it was an internal stud wall (without the cellotex layer) it would be easy as all I have to do is locate the studs (got a stud-finder...
