Stevey, Gareth, thanks.
The garage who diagnosed* also advised doing the work
(*not my normal one as engine work was anticipated and they dont do that)
I'll get them to do a though check over too, oils etc, whilst they have it apart.
I do use Millers Diesel Power Ecomax additive
Thanks for your thoughts, I bought the car in late 2017 with a full service history, it seemed to have been maintained 'no expense spared' at Audi dealership. Since then, it's been serviced by a local Audi specialist [ex main dealer "master" mechanic].