I thought other Octavia owners might have had this problem, but couldn't find anything posted. So here's how I fixed it.

The black plastic lowest portion of the rear bumper popped out from being connected to the body-coloured part immediately above, as a result of a light contact with a low barrier when parking.

After levering up the tabs, so that they would engage with the slots in the coloured bit, no amount of pushing or thumping on the black bit would get them to re-engage, as the coloured plastic just deflected backwards.

I thought I was going to have to take the bumper off, as access to the back of the coloured plastic is hampered by the steel reinforcing strut in the bumper. I managed to do if without by placing a suitable length of stout wood pressed tightly against the black plastic and secured to the ground behind the car, and then rotating an L-shaped steel bar from under the car against the back of the coloured plastic (just under the strut) as each tab position. It gave a very satisfying click when it re-engaged!

Hope that helps someone (and it's a reminder to me, in case I ever need to do it again 😁)

Thanks for your real life experience fix.

I suppose it's too late now, but some images or a vid clip would have really helped.

10 hours ago, CKO said:

some images or a vid clip would have really helped.

Yes, good point. I really didn't expect to be successful when I started, so I wasn't thinking of recording my efforts as a tutorial 😕

Would you believe I've done this again! Am I really the only person who has ever done this? Assuming I'm not, here are some photos this time, as suggested by @CKO

The first shows the problem (not as bad as last time, when a longer portion became detached with a larger gap).

The second photo shows the wood strut firmly supporting the black part, and the L-shaped tool used to push against the inside of the body-coloured part of the bumper above (a canal windlass!), and the lower part snapped back in.

15 hours ago, Octy-PHEV said:

Am I really the only person who has ever done this?

I've tried but failed miserably. Also in my case the gap is bigger and wider, though everything looks like it should fit back in its place.
Thanks for the pics! Will try once again.