Hi
I do not live in the UK but I can help you with your problem
I have been making, transforming and painting WWII vehicles and soldiers for the last 40 years
A burnt out vehicle is not a black object but it has a black-brown, rusty appearence.
The best effect you will obtain by mixing Revell pain matt orange (85) with Revell matt wine red(37) together with some oil paint nl: some burnt umber and Van Dyke brown. Using different quantities of the paints mentioned above on different surfaces of your tank or vehicle will give you the correct appearence of a burnt out vehicle
good luck
guy