There is a bit of confusion around smoke and the 2018 MoT update.

Firstly, you're right that the DPF smoke rule doesn't apply to your car. How can it? You vehicle doesn't have a DPF.

Secondly (and this is the bit that's causing confusion) the 2018 MoT update also introduced lower default limits for emissions - it is here I imagine that your car has failed.

The new limits for diesels are:

1.5m-1 (or plate value if lower) for cars first used on or after 1 July 2008
0.7m-1 (or plate value if lower) for cars first used on or after 2014.

Given that it was first used on 3 July 2008 it has to meet the 1.5m limit. Obviously, you can appeal the MoT failure but given the negative press around diesel emissions (and the thousands of deaths it's being linked to each year) I doubt the DVSA will rule in favour of a vehicle that breaks the new limits.

For a complete breakdown on the new rules, see: www.honestjohn.co.uk/news/miscellaneous/2018-05/uk.../