Quick question, trench blocks with plain ends 440 x 300 x 215 [ie not interlocking] I have been advised these dont need mortar joints beneath ground level, just butting together?
Yes that is so, Celon foundation blocks are laid like that, as can other foundation blocks, but I'm old school prefer my mortared perpends. T&G Trench blocks are just butted together.
If they are plain ended and not T&G as KIAB posted Then I would cement them together as you would a normal block/brick.
The large image I put up are plain ended blocks, but they have hand holds put in them to make handling easier,& are just butted together, but I would still mortar the end, & if they are like Phills image again I would mortar them together.