I see that none of the replacement fuel filters now come with the small o-ring seal.

I've had a few problems as current car came with ill-fitting item.

it's nominal 19mm, but does anyone have dimensions, or cross-section size, please?

3006 1.8 tdci KKDA

Do you mean the O-Ring that comes with this fuel filter here from ebay?

Well you are right. Most of the OEM filters are sold without the outlet o-ring, except Mann.  Even FoMoCo part (with Delphi stamp on) is without. From my experience do not use Bosch part since it is kind of loose fitting on a crucial spot. Other brands fit just right without o-ring replacement (Mahle, Fram).

9 hours ago, Luke4efc said:

Do you mean the O-Ring that comes with this fuel filter here from ebay?

No mate, not the large seal around the filter lid.

There is a smaller 19mm o-ring fits into the recess in the lid. It has to be the correct cross section to seal properly
 

9 hours ago, Ned_Mk2_1.8tdci said:

Well you are right. Most of the OEM filters are sold without the outlet o-ring, except Mann.  Even FoMoCo part (with Delphi stamp on) is without. From my experience do not use Bosch part since it is kind of loose fitting on a crucial spot. Other brands fit just right without o-ring replacement (Mahle, Fram).

Cheers Ned, I'll look for a Mann 😮  (Ooo Err mrs!)

Found one with a MAPCO filter

 

AWE NAW!!!   Out of stock EVERYWHERE!  😞