How to get to & tighten the gland nut on a single lever monobloc tap?

We have a single lever monobloc tap fitted to our kitchen sink, when the lever is raised to allow water to flow we get a goodly sized leak from around what I assume is the gland nut inside of the upper section of the tap, (the bit below the moving hood/handle).

Question? There is a small removable plastic plug (H&C indicator) in the lower part of the moving hood/handle, is there an allen screw behind it that holds the moving handle to the fixed part of the tap & if so what size? Or is there some other fixing that holds the two parts together & how does the tap come apart?

All of the above on the assumption that there is an adjustable gland nut in there in the first place.