Probably NOT the matrix at all, unless you have water in the interior footwells! (Check whether your coolant system does actually have sufficient water in it to start with!)
Most likely is that the "By pass valves" are playing up, and water is not circulating through the matrix.
If possible, check to see whether your car has had a new matrix fitted in the last 5 or so years.... most likely it has, so it will be safe to remove the By Pass valves, and return the car to how it left the factory.
By Pass valves were fitted by the dealer in the early days, because of a problem that was greatly exaggerated when a Matrix could burst, and dump hot water into the footwells
Later matrix were modified to be stronger under pressure, and eliminate the problem.
99% of cars don't run By Pass valves, because they're not needed, the Mk2 is no different
Thom
