Toby coughed up some electrons that declared:

How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...

Send me your email address (mine's valid) - I have a datasheet I can send
you on XLPE with actual and copper-equivalent CSAs.

Can't find the source though, otherwise I would have just posted a link...

Cheers

Tim