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.

I’ve just had an oil service required warning come on despite the oil being changed about 1500 miles ago. About 700 of those miles were covered in 3 journeys so I’m not sure what is going on. The oil doesn’t smell of diesel, but that’s hardly a lab report 21 plate D300 Westminster

If you have an hour or more to spare, it's all in this thread -

https://www.fullfatrr.com/forum/topic60839.html

As above though, it's basically oil dilution caused by failed DPF regens. What drives those is your driving - and less predictably, the length of journeys & when you decide to stop......

The good news is that oil changes are surprisingly easy.

As Alistair has said, read the thread he suggests, become an expert, buy the “Kot Kit”, and be in control.

All Hail to the Regeneration Gods! Let there be no failed regens! Dilution is an Abomination!

It’s not so much actual oil dilution per se, rather it’s the perceived oil dilution by the algorithm in the car, which to my experience is a bit paranoid.

With mostly urban journeys on our TDV6, we get between 0.1% and 1% dilution every 2500 miles, which is roughly every 6 months when we change the oil and do a lab analysis, this is because the owner can become as paranoid as the vehicle.
The “LIMIT” is 7% or 10%, depending on when your car was manufactured, but you’d have to be crazy or ignorant to regularly push it that far IMHO.

But please: discuss your experiences on the other thread, lest the Regeneration Gods be angry  V6 Vogue - “Less is More” - less to go wrong!

My last oil change was after 2500 miles. The oil analysis showed 1% dilution. So I will do oil change every second service light.

how does one find out the level of oil dilution on a 2020 l405 sdv8. RR owner

Buy an oil sampling kit from one of the oil labs advertising on the internet, I have used MyOilDoctor a few times, but there are others such as Millers etc.
They will post you a kit for about £40 with a small bottle, syringe and long thin plastic tube to shove down the dipstick tube and suck out about 80ml of oil. It comes with a prepaid bag and coded bottle label to mail the sample back for a detailed analysis, not just oil dilution. It usually takes about two weeks to get the report back.
It’s a really good idea to do this at least once a year for peace of mind. MOD have a “traffic light” system on the reports, so green means all good, yellow means monitor the situation, and red means imminent possibility of failure.
I do it every oil change, so every 6months or 3-4K miles.  V6 Vogue - “Less is More” - less to go wrong!

When is the best time to do this, I guess just before oil service ? RR owner

Yes of course, I do it at the oil service, (when you have plenty of old oil to sample).
At that time, you tend to record the miles since last service etc so its a good time to do it, and you don’t really need to mess about with a little syringe & tube, -unless you don’t DIY of course.  V6 Vogue - “Less is More” - less to go wrong!