Access to wall cavity, what insulation?

I have been investigating insulation options in my 1930's chalet house and have found that I can get pretty good access to the head of the external cavity wall at the top of the ground floor.
I can see right to the bottom and there is no insulation.
So this gives me a opportunity, do I fill with as thick a board of celotex I can, or do I use something else?
Any ventilation requirements in the cavity I need to consider?
WN

Rigid board insulation requires fitting tight to the inside wall. This will not be possible in your case. Likewise, full fill cavity batts will be impossible also.
Have you considered blown in CWI by contractor?
The contractors will need to ensure any through vents are kept clear.

Looking at the cavity again this morning its 60mm and the rigid insulation or batts won't work.
However what I have found the cavity has been filled with blown insulation in parts. However above a bay window there isn't any insulation.
I suppose that they couldn't get access to fill this area.
As this is actually a smaller area than I though, approximately 2m wide by 800mm high, I could fill/stuff with moisture resistant cavity roll, or CWI?
WN

Cavity roll? Does that even exist?
Blown CWI is your best bet. For the bay window, depending on your wall tie spacings you could try cutting some PIR board to fit from above. The chances of alternate courses of ties being perfectly plumb are slim to none though so would probably end up being a bit of a pigs ear.