Just got the Octy back from its 30000 service - and this morning that spanner symbol is lit up and flashing on the dash as I start up. Disappears after a few seconds. Does this mean the garage have forgotten to reset something?

Sounds like they forgot to reset the service interval indicator to me. I'd pop it back to them and get them to double check just in case it's something else.

I wondered if that might be it - you'd think they'd remember that, considering I've just handed over £186 for a posh oil change...

You would think so. My sister has an Audi TT. They forgot to re-attach a healight unit which nearly fell out completely under braking, and let her drive out with a tyre that was down to the braid. (inner edge). All they said was she might need a new tyre.

That "service" set her back nearly £600 !!!