They can be reached on the 601, 602 and 603 engines whichout pulling the manifold off, the last cylinder is had though.
You'll need a stubby 10mm ratchet spanner or 1/4 ratchet for the electrical terminal on the end of the glow plug.
You'll then need a deep 13mm (from memory it's 13mm) and a series of extensions and uni joints (3/8 drive or 1/4 if you've got a snapon set and they'll replace it if you break it
).
It's tricky but managable. Make sure when you tighten the 10mm nut that the cable doesn't turn and touch the manifold and short circuit......been tere, done that
later 4 valve engines 604(never seen one) 605 and 606 engines need the intake manifold to be removed
