Yes, I can now...
A "plume kit" or as Vaillant describe a "vertical termination kit" has been used to raise the
flue intake from it's termination point to 2.5m (it is not a flue extension) however the
flue exhaust still terminates below ground level which is why it has been pulled up. As described above a balanced flue attached to your boiler contains both the intake and exhaust ports, the plume kit will need to be replaced with an actual flue that can terminate at 300+mm above ground level as long as it is caged or above 2m where it does not.
You can see from the diagram that the vertical termination kit is a lesser diameter from the actual flue exiting the boiler, that's because it is only extending the exhaust port meaning in your case the intake (at termination point) will still be below ground level with the potential to get blocked.
Your confusion (and mine a bit) lies in the belief the plume kit is the same as an actual flue and therefore terminates at a legal height, it does not it only partially does via a plume (exhaust) extension kit. Hope that clarifies things.
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Corrected, sorry wrong way around.