[Closed] Loft Conversion Question

We have a loft conversion.

This was done as a dormer on the side of the roof for the stairs and with velux windows front and back on the original roof - so the actual rooms in the roof have sloping walls and no dormer.

When originally done the roof was split into two bedrooms (thick black line);

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there's a water tank in the loft above (blue box). The dividing wall is a standard stud partition but has a supporting brick wall underneath the joists it resting on and the tank is directly over it. There's no big metal beams or joists in any of the roof voids.

There's also a small toilet and basin room in the converted space.

We'd like to do a few things;

1) remove the dividing wall between the two halves of the roof to make one large room

2) introduce some additional velux windows to the rear (maybe 2 or 3 long thin type ones)

3) change the small toilet room into a shower room by moving a stud wall.

Initial question I have is whether the stud wall with the tank above is likely to be structural? I'd rather not have to move the tank (it's currently in the middle of the roof) but I coudl move it next to the chimney breast if that helped.

I assume I'll need building regs for introducing new windows but will I have to upgrade insulation in the roof void to modern standards (conversion is at least 15 years old) if I make substantial changes to its layout etc. When we bought we were able to get a building regs note out the council but they have no other record so I guess change of layout would be 'hidden' from them in terms of reporting to any purchaser.

House is a '30's semi.