It's far easier to take the whole bumper off ... basically two bolts
through a slot.  Once it's off the end-caps, IIRC, are held on with
self-tapping screws (naff) and if they're loosethey're unlikely to
tighten up properly anyway as they mostly (Mine have three times) just
been pulled out, creating a large hole in the sprue plastic of the
end-cap.  Araldite into the hole, then drilled 2mm dia when dry is a
reasonable 'cure' short of new caps.  There's a metal triangular plate
that they screw through and this is also, invariably, bent a tad out of
shape, so just twatting them while still attached doesn't bend it
enough to put it back properly.

They can be done with the bumper in situ, but it requires a lot thinner
arms with a lot more dexterity than I have.  Whole bumper off is pretty
easy actually ... getting it on again 'and lined up' is a tad more
awkward, but still far easier than attempting the end caps on the
vehicle.  ;)