Light behind bathroom's drop down basin

I'd not noticed this before on our Kemerton XL but when we were away recently the sun was bright and shining on the side of the van where there's the toilet compartment. Dropping down the basin I realised that the sunlight was illuminating the back. In our Boxer panel van there are still all the existing windows down each side that are still there even after A-S have converted the Boxer van into a panel van motorhome. What the illuminated basin enclosure seems to indicate is that inadequate insulation has been installed between the back of the basin surround and the pre-existing window.  It is probably a factor why we along with other owners of a Kemerton XL have reported the bathroom area in winter can be colder that the rest of the living space. There's not much I can do about this as I'm not dismantling the basin surround.

Last year I thought I might in future have to dismantle the surround. The bathroom tap kept coming loose which A-S finally sorted with a replacement tap, the dealer had two goes at fixing it. The original mixer spout was too stiff and would cause the single fixing screw of the mixer tap to work loose.