We don't; the member has to, and it is a slight problem. The member takes responsibility. If she or he puts in for a 60- or 90-minute debate, it says very plainly that you shouldn't do this, and if you want to say which other members are putting in, please put it in on the form.
You can't force them to turn up; you're absolutely right. I guess maybe about once a week or once a fortnight, we do get what's meant to be a longer debate, but even with a long speech from the member starting, other members have not turned up in the event, and the thing falls short. We suspend so as to know when we're starting the next one with a new minister and new cast.
I don't think there's a public black mark against the member, but there's certainly a private one to note that they are not meant to go for the longer debates unless they think they can fill the space, not just with themselves, but with colleagues.