[BrendanP]

I've changed the gearbox oil a couple of times on my 2015 diesel, and never bothered to change the crush washers. It could be they were stuck to the plugs and I didn't notice them. This time I will change them, and they seem to be a ridiculous price on-line, but my local Kia dealer supplied them for the princely sum of £1-01 each.
Part number is 17511 16000.
For the record, they are aluminium, 1mm thick. 23.7mm external diameter, 16.2mm internal diameter. This might be useful for anyone trying to find a 3rd party crush washer as an alternative to the original part.

[The Gonz]

Thanks for the hot tip, Brendan. There really should be no reason for aluminium washers of any type to be expensive.