How deep do the back boxes need to be to install the Powerline socket?

How deep do the back boxes need to be to install the Powerline socket?

The Power Ethernet T1000 Socket fits a standard UK two-gang 35mm deep metal back box. A matching plastic space (aka a shim) is included with the product to make it easier to install in shallower backboxes or where there are many cables.

The Power Ethernet T1101 Mainline Premium Data Socket connects directly to the Mainline track system.