Improve insulation in dormer

Our dormer semi D built 2002 is crap at retaining heat in the upstairs rooms, i.e. the dormer part. The main problem seems to be the crawlspace behind the rooms upstairs.
From much research I have determined the following:

It's extremely difficult to retrofit insulation into the crawlspace, that is behind the dwarf walls upstairs.

The main problem is airtightness rather than insulation levels.

Curerently there is fibreglass insulation between the studs of the dwarf walls and along the floor of the crawlspace out to the eaves. I would like to improve on this if possible and would like an opinion from someone in the know...

Someone advised me to:
1. Cut small blocks of insulating board and fit into the spaces between the joists in the crawlspace below the dwarf wals and seal around the gaps, if any, with foam. This is aimed at stopping the outside air blowing between the ceiling downstairs and the floor upstairs which, I'm told, removes a lot of heat from the house.

2. Fit ecoquilt or multifoil wrap along the back of the walls on top of the insulation already there and seal as best I can.

3. Add another 200mm of roll insulation on top of the existing insulation in the 'floor' of the crawlspace and get some extra and fill the eaves, leaving ventilation free.

Has anyone done any work in this space in a dormer and if so, how did it work out?

I'm hoping someone can give an opinion on the insulation suggestion I have had from an installer.

The plan would be to use spray foam from the wall plate, along the floor of the crawlspace and up the back of the dwarf walls to meet the slope. Vent cards will be fitted to the eaves and to where the dwarf walls meet the slope of the rafters in order to maintain the ventilation right from the eaves up to the attic. This will sort out most of the draughts in the upstairs coming from behind the walls.

However, behind the sloping roof upstairs there is fibreglass 'stuffed' in and I would like to improve on this. I was thinking I could remove the fibreglass and replace with a better insulation like a rigid board or similar maintaining a 50mm ventilation gap, or should I just leave the fibreglass in place. (there appears to be no ventilation gap there at present)

Then finally I would upgrade the attic floor insulation to the required 300 or 400mm fibreglass roll or rockwool.

Any thoughts appreciated.

Very similar problem with our dormer. How did you solve your problem colin79ie?