We've used au pair world successfully a few times.
Run through your day, telling them exactly what you expect from them. What time they need to be up dressed and working. What time the kids need breakfast. What they have to do next....everything. You want to be near-enough putting them off coming to you. See what the response is. (You want your au pair to know exactly what they are expected to do when they arrive, so there's no point in hiding anything....will only lead to problems when they discover the truth).
Ask about them, their family, their childcare experience, why they want to be an au pair, why they want to come to the UK, if they have ever been here, how long they want to be an au pair, what they want to do after being an au pair.
You want them to be asking you about your kids - what they like doing, what their personalities are like. If they barely mention them take it as a bad sign.
You'll gradually cut them down to a small handful who consistently come across and friendly and genuine on email. You can then call them to chat, have some questions ready. A great time to check out their English.
It's a long process but worthwhile to find the right one.