2. Leave car to soak overnight in a torrential rainstorm.
3. Go shopping in car for Christmas presents and the first batch of the Christmas food. Get thoroughly damp in the process.
4. While unloading the purchases, notice that the thick grime is nicely loosened and ready to transfer itself eagerly to hands and clothing.
5. Dump purchases in the front hall and run a large bowl of warm water. Make the water frothy with wash-n-wax car shampoo.
6. Grab a soft, plastic-bristled scrubbing brush and attack car with soapy water.
7. When the entire car has been scrubbed (apart from the roof, 'cos I forgot that bit and anyway it was more or less clean), throw a couple of bowls of clean water over it to rinse.
8. Leave the rain to finish the rinsing.
Next day you will find a car of dazzling cleanliness (at least compared to how it was before) and though it's by no means perfect, it is now presentable enough to take anywhere and can now be touched or leant on without getting covered in black gunk.