MM wrote:

I've made the first stage, i.e. the copper tube, about 3 metres of it
in a "snake" pattern. This will be mounted on a south-facing wall. Can
I feed the hot water (on thermo-syphon principle) into a black plastic
cold water tank as used in the loft? Or will this plastic release
toxins when containing hot water?

The biggest thing to worry about is that the tank will, if the water
gets hot enough, collapse and release hot water. You really need
something that can take near-boiling water, and the tank needs to be
insulated for best effect and closed (but vented) to prevent heat loss
by evaporation.

My concern here is that you've shown a slight lack of clue from the
outset and now it sounds like you're building up to kill someone,
possibly yourself.