@Michelle_B I presume you're listed for telephone mediation as that seems to be the default now? It's important to understand what the process is and, maybe more importantly, what it isn't.
1. The mediator won't make a decision. In fact, by law, can't make a decision. Their sole role is to try and broker an agreement between landlord and tenant. If that works, great. It if doesn't you can go to the next stage which is a tenancy tribunal.
2. By serving a termination notice you've already asked the tenants to leave. They have refused. Obviously they're likely to refuse a similar request from the mediator! Why wouldn't they? So, how to break the deadlock? The only variables in the equation are if and when the tenants move out and whether any financial incentives are offered. This is the space within which the mediator can work.
3. You need to understand the timelines. If mediation is unsuccessful, you go into a queue for a tenancy tribunal hearing. That seems to take about 3 months or so. Perhaps a month to get a determination order, plus whatever notice is specified on it. Say about 5 months in total all going well. Then if the tenants don't move, you need to go to the District Court to get an enforcement order. Say another three months. So you're looking at eight months, maybe more.
4. Now back to the mediation process. It might be possible to reach agreement that the tenants will move out on a specific day, say about 3 to 4 months away. If you both agree on that, then RTB will write a determination order that confirms it. That brings you directly to the endpoint you would otherwise take longer to reach (maybe much longer) if there's no agreement. They can still ignore the determination order and force you to drag them through the district Court but they're likely to get far less sympathy if they've already agreed voluntarily to the terms of the determination order.
5. The tenants have an incentive to agree too, as they know they have security for a few months while they search for someplace else. Nothing to stop you offering a partial refund of rent if they expedite their move out!
While that's fine in theory, mediation like politics is the art of the possible. A compromise that gets you a result might be better than a principled tilt against windmills that gets you sucked into a long drawn out process. Time is of the essence here and it's running against you. Not fair but that's the system.
