I agree that 20% off is a noticeable reduction but £850 is still rather eye watering! What’s so good about these tripods?
Gitzos are better made, simple as. They only make top quality tripods and every one is best in class (and invariably the most expensive). You can't say that about any other brand - Benro, Manfrotto, Sirui etc all have some excellent models, and probably better value, but never better performing and they also produce some cheaper, less good models. I've probably tested over a hundred tripods over the last few years for the photo magazines I work for, and you can tell a Gitzo blindfold just by giving it a bit of a shake-down.
Best way I can put it - tripods come in broadly similar size categories, 0-series, 1-series, 2-series (usually the most popular), and then for heavier calibre stuff there's 3, 4 and 5. A Gitzo will typically outperform other tripods one category higher, often more.
Buying a used Gitzo is certainly a good way to a bargain, but you need to know what you want, understand all the model codes (there are dozens of them, all with numerous permutations), keep scanning the ads and be patient, and be comfortable buying unseen by post. Gitzos can always be serviced and repaired, but that's not cheap either.
ps Every photographer who's serious about tripod working either has a Gitzo, or wants one. There's a kind of inevitability about it...
