My heater matrix leak seems to have been cured by K-Seal, which did just what it said on the bottle. I shall be interested to see if it really lasts the lifetime of the car, as advertised.

However, before I did the treatment I thought I'd flush the system as the coolant was a nice, deep rust colour. Flushing though the expansion tank did no good at all - the water that drained out of drain valve (which works fine on mine) stayed a deep, rust red, so I decided to clean out the expansion tank, which had mineral deposits in it - clearly visible.

Pretty straight forward to remove -it's held in by two rubber lugs that locate into the side of the engine compartment and a screw (one of those star shaped ones) between the tank and the ECU cover which was hard to get out until the ECU cover was removed. Three hoses: two small and one big one came off easily and I replaced the spring clip on the bottom one with a jubilee clip. Then I took the tank into the garden for a good sluicing with the hose. Black water gushed out of the bottom and ten minutes later - no kidding - of sluicing and shaking and sluicing it was still gushing out. Fifteen solid minutes of sluicing plus a good shake up with sugar soap (all I had) and it started to clear.

I suppose the moral of the story is that the traps and baffles at the bottom of the expansion tank must work - simply because they had trapped such an extraordinary amount of gunk. I have to say, the matrix leak apart, this has never affected either the heating of cooling of the car, both of which work fine. It will be "interesting" to see if I've stirred up problems for the future though.