Just had the check engine light come on, and OBD checker says P0420 which is an error from a catalyst sensor. OK to drive for  ~100 miles tomorrow before I can get it in to a garage? And while we're at it, anyone else have experience of this on a Mk8?

You sure can drive it, it will just pollute a lot xD

As a follow up to this, I took the car to a Ford service centre to be checked - they said it was a fault with the catalyst and they replaced the catalyst under (extended) warranty. I'm a bit dubious whether there was genuinely a fault with the catalyst, or if they just read the same catalyst error code as I did and swapped it out. All seems fine now though.

Tbh I think it really was the catalyst, I can't see anything else, other than maybe the sensor, that could have caused the problem.

Quite often the sensor will throw this because a combination of factors will push it over the trigger point.  Could be a slight leak somewhere.  For the actual catalyst to fail after four years is unlikely. Unless you've done stellar miles OP?

Sensible/best practice is to reset it and see if it occurs again.  The Ford SC will simply have done the option that makes them the most money.

Yes, I thought it more likely to be a sensor/wiring issue rather than a failed catalyst at 4 years (even having done 80K miles).

I did reset the fault and it came straight back.

That's the problem with this type of system.  You don't actually know the cause, but sensors and wiring, by their very nature, don't survive well in that environment.