Is 5cm deep really the recommended depth? I tend to just go as deep as
the plaster is. Once I hit brick that is it.

And is this just a recommendation or compulsory.


For cables not running in the "standard" routes, it's mandated by the
Regs: 5cm (2 of your earth inches) is what the regs-writers consider to
be deeper than a casual picture-hook or small-fitting-wallplug will go.
If the cable runs in the "standard" routes (once again: a 6-inch/15cm
band vertically and horizontally from each visible fitting, and a
similar-width band around the top and side BUT NOT BOTTOM corners of
each room), the 5cm depth is *not* required.

The other approved way of running cables in the non-standard routes is
to provide hefty additional protection: in practice that would mean
serious metal (which would need to be earthed), such as heavy-duty
conduit or trunking. Galvanised capping isn't protection enough - it's
easily penetrated by a nail (which is how you usually fix it!).

HTH - Stefek