A cry that may be heard from Wimbledon this week, but in this case it was the cry of the Ordinary Van reminding me to book it in. It's been an 'interesting' journey, more eventful than I predicted:
viewtopic.php?p=456579#p456579
The service is booked for next week so I thought I'd have a look round in case any items need bringing to the attention of the service department, apart from the rust stains described above. There is still fresh oil on the bottom of the transmission housing, and although the driveshaft seal still looks a bit moist, I don't think it's coming from there:

viewtopic.php?p=558198#p558198
At the time the oil seal was changed, the oil drain pipe from the turbo was also changed, so I crawled underneath for a look. There was a drop of oil collecting on the underside of the air inlet hose to the turbo (arrowed), and in the picture it looks smeared because I ran my finger across it to check that it was oil. Since there is nothing above the turbo:

Library photo (mine)
I would say that the only way the oil could be there is if it is collecting inside the hose and seeping out, although of course the hose should be airtight. There was also a small amount of oil on the clip on the hose to the intercooler (under the tail of the arrow):

For those not familiar, this picture was taken from below, looking up the back of the engine, the drain pipe is the one in the foreground, with the bellows section.