My L405 had a service about 7 months ago when I bought it - a simple oil and filter change. I've had a warning on my dash saying that a service is due in 12 days - but the InControl app says a service isn't due for another 12k miles. Is this just a case that the car wasn't updated post the oil/filter change, or am I actually requiring a service?

Sadly not, these cars can suffer from Oil Dilution, which means if you do lots of small journeys, then the service light will come up sooner than the required interval.
I get about 5-7000 miles between oil services, be warned if you ignore it.
It's a fail safe for a badly positioned DPF.