I may also put a black-painted length of copper pipe inside the bottle, to act as a heating element, if I can get my hands on a length of copper pipe at a low cost.

I also have no black paint.

Bubble wrap would be an admirable insulator for this application.

I have some bubble wrap.

You could use a mirror(s) to reflect additional sunlight onto the bottle.

As suggested, I added bubble wrap to the back of my solar-powered water heating bottle.

After one and a half hours in the window, the water in it was still cool.

So, I followed YouTube's guidance and put it inside a larger bottle.

After an hour and a half, it had warmed up noticeably.

But still not very much.

My next step will be painting it black, to see if that makes any difference.

Once I have access to some black paint.

In other news, I used 0.28 units of gas yesterday. The lowest since my experiments began.

Tomorrow will be my big test, with temperatures outside expected to plummet.

I have now have a black-painted bottle.

Now all I have to do is wait for the sun to come out.

I have a mystery draught in my bedroom. I know about it because, every time the wind blows outside, the sheet of cooking foil I put over my bedroom door's little window makes, "I've been hit by wind," noises.

However, I can't find out where the draught is getting in from. I have been all around the window frame and the door frames, with a candle flame and it showed no signs of being blown around by wind.

So where is the wind getting in from? Where?

The Call of the Wendigo wrote:

my bedroom door's little window...

The Call of the Wendigo wrote:

...makes, "I've been hit by wind," noises.

Perhaps tin foil isn't the best thing for the job?

It's for preventing radiant heat loss.

Behind it, I have a layer of bubble wrap, to prevent conductive and convective heat loss.

Have you considered steadying, supporting, bolstering and soundproofing the overall superstructure with a layer of gaffer tape?

I am considering it.