Rueben Alsop

Very much so, and having cellar access makes you lucky. Mineral wool batts held up with netting stapled to the joists is the standard approach — leave an air gap and don't block any subfloor vents, because those keep the timber dry.

Completely normal and it's a moisture cycle, not a fault. Timber gives up water when the heating is on and takes it back in the humid months, and it moves across the grain far more than along it. On a 150mm board a 5mm swing is unremarkable.

Do not fill them with anything rigid. Filler, resin or wood glue in a gap that moves seasonally will either crack out or, worse, hold the boards apart so they can't close in summer and then split them somewhere else. The traditional answers are slivers of matching timber tapped in and glued to one side only, or a rope-and-caulk method, or accepting the gaps and dealing with the draught underneath instead.

Honestly, in a room you want warm, insulating between the joists from below and draught-proofing the perimeter does far more than filling gaps ever will.

Definitely hollow either side of two of the cracks, solid on the third.

There's a bedroom above so no loft access. Do the washers show through afterwards?

If any of the plaster does need replacing later, use a lime-based plaster rather than gypsum on a solid brick wall. Gypsum traps moisture against the brick and you get a repeat performance in three years.

Can confirm from experience. Also it makes the sash harder to open, and a window you stop opening starts rotting.

Worth adding: taking in more than about 2 inches at the back seam starts to distort the pockets and belt loops. If you need 4 inches you're in the wrong cut, not the wrong size.

This is the real point. My partial rewire cost me a second round of plaster repairs and redecoration three years later, and by the time I added that up the full job would have been cheaper.