Total lack of urgency and communication

We purchase a diamond engagement ring from Browns in Somerset Mall which we collected on 22 January 2016. Nine days later the main diamond fell out and on returning it to the store were told the claws for the two adjacent diamonds were also loose but that it would be repaired and returned within the guaranteed ten working day period. The ring had to be sent up to Head Office for assessment and left that Thursday afternoon with the promise that I would be phoned on the Monday. Needless to say no call was received and I called the store and Head Office on the Wednesday. I was informed it would be sent for assessment the following day with no explanation as to why it took six days from head office to assessment centre. I kept stressing that this was an engagement ring and my fiance was extremely upset not to have it at our engagement party or to show to her friends. The following Monday I was contacted and told the ring could not be fixed and they would replaced with a similar one and would be with us within the ten working day return period. Since then I have heard nothing. The store are waiting on Head Office who are trying to get a ring from another store - 18 days later!