Kev - Had a gander in the cupboard under the boiler yesterday and, yes, there is a drop out vent directly under the boiler (boiler is on a small wooden shelf on legs 3" high). The drop out vent is of the 'top-hat' variety as opposed to just a flat grille - about 2" in diameter and made of beige plastic. Another exists under the cooker.I will have a look at ours tomorrow - it looks the same as yours - and see if I can find a low-level vent. Our sits beneath the wardrobe, behind the Truma heater/fire, in its own little cubby-hole. The boiler itself is completely room sealed.
You've got me thinking now...........