In reply to haworthjim:
A few years ago I lost my NBC, and as a fairly new and younger driver, struggled to get reasonably priced insurance. Black box option was cheaper by a few hundred quid. Bloke came round to fit it, no dramas. Tried to drive how they wanted, checked my online report once around 6 months in and was on silver medal, apparently. Didn't bother checking it again. End of year they made no mention of it, offered me a renewal that was pretty high. Went on compare website, found a cheaper price without black box, and they didn't ask if I'd had a black box. As far as I can see they don't share that info.
IME get one for cheaper premium, ignore it, swap companies at years end. UNLESS... I hear some companies will penalise you mid year for crap driving, so check that.