So, I picked up a 6yr old RRS this week from a dealer down south. As I was driving it home, it broke down 😭

I won't name the dealer as they're being relatively helpful so far. They've arranged to pick it up next week for investigations. I just want to have an idea of what went wrong, and how long it might take to get sorted.

It's a pre-facelift 2018 RRS (is that MY 2017?). Engine is SDV6 with about 66,000 miles. Most recent service was just 500 miles ago - is there any risk this wasn't carried out properly?

Breakdown symptoms were as follows:
Motorway cruising, started to hear a kind of flapping, as though a scarf was stuck out the window, or like you get on concrete motorways. Turned music off, and it got steadily louder so I tried some diagnostics of sorts. Pretty sure it was related to engine speed, not vehicle speed.
This makes me think it's maybe a belt of some sort?
Anyway, I'd made the decision to pull off at the next services, but before I reached them, the engine cut out. Can't remember if there was a clunk or anything, I was focused on getting safely to the hard shoulder.
The D on the cluster was flashing, then came up saying something like "re-engage gear and try again". My assumption is this was the gearbox trying to cope with a stationary engine.
While coasting along the hard shoulder, I tried to restart the engine, which didn't work. I tried it again once stopped, also failed.

My thoughts are that it's something major, I hear lots about crankshafts, timing belts and HPFP belts. Do any of these sound likely?

All I need to know really is how long until I'm able to drive it again. It's a beautiful car, drove like a dream for the first few hours.

Possibly hpfp belt. Would be fairly noisy from inside.