This is a possible solution to Gary Millang's VW Freezing Problem.
Many years ago I had a similar problem with a US car that should relate
to Gary's problem.  The problem turned out to be water in the fuel
tank/line.  Water has a higher specific gravity than fuel and is
relatively immiscible.  Water also collects in the lowest point of the
fuel system.  When the temperature is above freezing (32 degrees F.) the
water is a liquid and does not interfere with the passage of the fuel. 
However at temperatures below freezing the water changes state from
liquid to solid and this can restrict (evidenced by loss of power) or
completely block the flow.
The water can come from a filling station tank or be the long term result
of condensation.
There are 2 ways to fix this problem.  1.  Remove the fuel line at the
lowest point, drain it and reinstall it or 2. Add an alcohol based
solvent that will dissolve the water into the fuel and be carried
through the engine and exhausted.  These alcohol based solvents are
available at auto supply stores or denatured ethyl alcohol could be
used.
I hope that this works for you.
Another comment:  A water soluble oil can be added to the glycol-water
coolant to prevent corrosion in the cooling system, particularly the
cylinder heads, and to lubricate the water pump bearing.
Al Coha