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Thank you Old-Geezer - I had a brain storm at 3am this morning! - switched on computer, very, very slow to boot up but then I deleted my entire browsing history (on the list was some plug in and as I kept getting a warning about this each time my computer failed to respond, this was almost certainly the problem) Anyway, for the past couple of hours it's been behaving rather like a spring chicken and I'm delighted to have my computer back to play with. Many thanks for your help.