Just in case anyone else gets this (or a similar) problem, during our test upgrade to v7700 we found that the upgrade failed when trying to drop a table index which wasn't present.

The full error message was: Cannot drop the index ‘WorkOrder.WorkOrder_IDX6’, because it does not exist or you do not have permission.

Following investigation from the SC+ Support and Development teams it was determined that this is a problem which can affect some MSSQL users.  A patch has been produced, v7701, which has resolved the problem on our test system.