There's an electric, auxillary pump to circulate the coolant through the
heat exchanger. It's located on the driver's side of the engine, in front,
just below the thermostat; it's about the size of a grapefruit and has two
hoses coming out the back. The motor probably needs the brushes replaced -
but Jaguar doesn't offer them. A new one costs in excess of $300. You might
try to take it so a small electric motor repair shop; perhaps they have
some brushes to fit.  I live in Phoenix and can handle those 45 degree
cold mornings without a heater.