Lobster wrote:

I need to run a 10mm earth bonding cable [...]

Do I need to do this using one unbroken length of cable, or am I allowed
to make a crimped joint to connect two shorter lengths?

A joint is allowed. Joints in protective conductors like this are
subject to the same rules as any other joints - i.e. they must be sound
and protected from corrosion, etc. and must be accessible if not
crimped, welded, brazed, soldered or resin potted.

An in-line crimped joint will be fine in a reasonably dry location. I'd
protect it with overall heatshrink sleeving, although that's not
actually required for sizes of 10mm^2 and over. Bear in mind that
10mm^2 is beyond the range of the usual insulated crimps so you'll need
to beg, borrow or hire the appropriate heavier duty crimp tool and the
correct crimp 'lug'.

--
Andy