The Apprentice wrote:
What you need to block off your window is Kingspan the material they use for insulating roofs, it has silver foil on either side and comes in different thicknesses up to four inches.
Try steering away from cardboard because it will retain heat and transfer it through into the room your trying to insulate and bugs also love to eat the glue in there.
For shelving the best material is concrete pathing slabs which retain their coldness unlike timber which will retain heat.
Try and find some concrete building blocks and start with a nine inch block on the floor and then a slab and some more blocks and so on until you have built yourself a cold tower.
Thanks apprentice!
I hear what you're saying re cardboard, but it's all I can afford at present. It's also really tricky to get up and into the opening to fix anything to it, which is why I opted for this for now. I had thought about painting it out as dubs suggested when I have more cash. I also thought about just using standard foil as a temporary thing, but the window faces the street and I didn't want people to think I was growing something up here that I didn't ought to.

As it is, I've double wrapped a slot of cardboard to fit the actual glass panel of the window with lining paper/the vinyl stuff you can buy for tables, so I'll see how that holds up for now.
The larder was already here when I moved in, but had no door or shelving. I just stuck my fridge in there for many years, as it was a bit of a wasted space but decided to reinstate it recently.
It already has a cold slab built-in, and Ive added some wooden shelving above that. I hear what you're saying about concrete though, so thanks again.
There would have been something similar built into the window that was there originally, but whoever replaced it just put a bog standard glazing unit back in. It doesn't open but does have a ventilation slot which I have not blocked. There are air-bricks to the external in the section under the concrete slab, which is in the middle, and another one at the top of the cupboard above the window, so it's quite well ventilated anyway.
Again, until they replaced the window, it would have been a perfectly serviceable storage space, particularly as the kitchen itself is weeny. The landlord, whilst doing this for some reason refuses to remove the actual brick walls of the larder and the concrete slab, so I thought I'd make use of it. I'm very proud, even if I do say so myself. Plus, it makes my Spam look a bit less naff.