I bought my fiancée's engagement ring and both our wedding rings from Quality Diamonds and was very happy with the overall service and price. Having done the initial walk around Hatton Garden, I decided to go with Quality Diamonds because they were not pushy and very flexible and generous with their time, not mentioning discreet (they operate out of an office rather than a walk-in shop, so you do need an appointment to see them, but that is well worth it when compared to the feeling of being treated like a number at some of the shops on the main strip). And my sense was that because of the lower overheads, they were comparable - if not cheaper - than the busier shops on Hatton Garden proper.