The red stuff could potentially be the cause. There are reports of red "oat" coolant not working right in older cars that aren't designed for it and forming sludge.
I'd have a feel round some of the coolant pipes as the engine warms up. If it is taking ages for the feed and return to the heater matrix to warm up, I'd look to a partially blocked matrix. Drop the coolant, fill with water and flushing chemical as per instructions on the product you use, thoroughly flush the system in both directions with a hose, then refil with the correct coolant