[KahiroSamo]

the corosion inhibiters in the standard antifreeze (ethelene glycol, the green stuff) break down after a couple of years. that is the primary reason for changing out the antifreze every couple of years. this also helps prevent the galvanic action that can eat away at cooling systems. it's probably not a big deal if all the metals in your cooling system are the same (ex. aluminum block, heads, radiator, and heater core) but can be rough on engines with disimilar materials like my brothers 64 lesabre (cast iron block, aluminum heads, and copper/brass radiator/heater core). if you overheat the engine on a vehicle with aluminum head(s) you will most likely blow the head gasket(s). ask your mechanic how much the charge for a head gasket replacement is. you'll see that the 20 quid for a flush and fill is cheap insurance, and yes if the water is clear you do need some coolant for the corosion inhibition.