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?

NB: The tank will be mounted initially on outside wall, above the
copper tube "snake".

Or maybe you have suggestions for alternative tank suitable for hot
water?

The cold water tank I'm looking at (on the B&Q web site) is the Titan
Wizard 25/15 Rectangular Water Tank KM15 Black.

MM