Is the quarantine/self-isolation on return from overseas monitored in some way?

[CarryOnFestiveNamechanging]

Heated debate with family members who plan to ignore it, with a ‘nobody will know’ attitude. Returning from South Africa.

Hopefully, but sadly I think they are relying on people doing the right thing.

Haven’t we stopped flights from SA?

There are repatriation flights for people stuck there.

Not in my experience with relative flying and staying with us just before Xmas (different country but not in the safe list). Now back to their own country and quarantined in a hotel for 7 days, possibly longer.

In a word, no.

Think it might be from SA

It’s required by law so “someone” coukd report them to the police. Depending on resources available they might pay a visit

Sort of. You might get a phone call. But since that can be a mobile number, you could be anywhere when it's answered.
If caught you can get a fine.

One of the biggest issues with controlling covid is that we are relying on everyone to do the right thing and a lot of people don't give a shiny shite about anyone but themselves. Hope you gave them a piece of your mind OP, its beyond selfish