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Many thanks for all the responses. Your advice much appreciated. I take the point re avoiding a cheap Chambers as I did fall to the temptation of a 'bargain' and regretted it. I've also noticed that in many shops the dictionaries are sealed, so you can't look through them to do a quick check, which is a pain. But I suppose that's done to prevent them getting mauled and tatty, and they are quite pricey, so it's understandable. So, Chambers it is.
I google quite alot when I'm really stuck, and use this fantastic web-site, but I prefer to troll through reference books first.
Thanks again one and all.