The other day when I was driving along I suddenly noticed that steam was comming from my bonnet,, so I pulled over to find the the expansion tank for the water was overflowing,,,, it stopped after a while,, when I got home left her until morning to cool down then topped back up cause now she was low,, happened again last night, the end result she was very low,,, what on earth could be causing this,, could I have a block somewhere that is causing water to return to the expansion tank,, Am a little concerned,, maybe my rad is scaled up or something,, any thoughts?

how many miles has your car done? have you worked on the cooling system recently? also look for damage or distortion on the radiator. might even pay to check the thermostat first, too. what does your temp guage say?
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Sounds like it time for a new thermostat mate. Its probably got stuck closed

Maybe due to an air lock? Get the engine up to normal running temp and squeeze the Top Pipe a few times and see if you get bubbles in the expansion tank. Keep squeezing until you don't get any bubbles.

Just an idea.

Mark