The professional guy in Grahams (who are main dealers i believe) spent an hour looking through the boiler diagrams online and all the brochures and finally was told by someone from WB the part number of this "adapter" . They asked if the flue entry pipe on the boiler is above or below the top of the frame ( it is below so needs the adapter to bring it above the frame ) the flue kit elbow is then connected to this adapter.
The flue kit contains the elbow and straight pipe to go thru the wall. Nothing else.

The elbow does not connect straight to the boiler because it would not clear the top of the frame.

WB said some boilers have this adapter included "but not necessarily". They gave the guy a number.
He then spent another 15 minutes looking for the number in the brochures to identify the part. It did not exist.
I am not making this up!
I expected a call on Monday confirming they had this part on order but no luck.

The boiler manual shows a flue connector which is about 5 inches long and has a screw clamp in the picture but it was not with the boiler. It does NOT come with the flue kit. I would say it is more like the flue diameter not 2 inches but hard to say.
The model is the 26 which is the smaller one.