Thank you for responding.

I'm in Australia and the machine sold to me by an authorised Mac reseller had SnowLeopard installed. Not being a Mac afficianado, I didn't know that the upgrade to Lion was imminent or I'd have delayed my purchase by a month or two. I do now know that within weeks of my purchase Lion was introduced; my intro to Mac at the Apple store in August was all on Lion.

In any case, I've now learned from Applecare that the later updates to Snow Leopard are the cause of the problem and that a fix should soon be released. Nice of Apple not to let on until pressed! I'm now scared that Apple doesn't support superseded versions of its software for very long. Even Microsoft is still supporting my old WinXP, SP3 - but not for much longer. How old is that?

However I do love the hardware and its build quality is superb.