Cheers guys. I've noticed a small pool of coolant in the driveway, directly underneath where the expansion tank is located, so assume this is the problem item. On each occasion I've used the vehicle recently (about once every 2 weeks), I've had to top-up the expansion tank. First time it took only about 2 cups but most recently it took almost 2 litres of water.... I'm just praying it's not the dreaded transfer pipe job...
And as for your comment/suggestion Jopecasa - I'm pretty careful when it comes to playing with nipples - always treat them quite gently...

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08/06 build 4.4i Stirling Grey over black