On 10th January National Grid revised the GBA Trigger level as a result of the Short Range Storage stocks being within 2 days of the Safety Monitor level.  A further revision of the GBA Trigger is now being implemented to reflect the latest supply position, notably the sustained and significant level of Interconnector (IUK) exports. 
In addition to changes to the flow assumptions for IUK we have also reviewed the supply performance of all other major supply components.  The outcome of this review of supply components is to reduce the contribution of IUK imports from 20 mcm/d to zero. This reduction is part offset by marginally higher flows from the UKCS and Norway resulting in an overall reduction in the GBA Trigger level from 428.7 mcm/d to 414.1 mcm/d.
The GBA Trigger Level will remain under regular review.  The Safety Monitor levels have also been reviewed and no change is required at this time.

Full details of what a Gas Balancing Alert entails and how the triggers are calculated can be found at:  http://www.nationalgrid.com/uk/Gas/OperationalInfo/GBA/
However, a brief summary is detailed below:
The purpose of the Gas Balancing Alert (GBA) is to indicate a potential requirement for demand side response. It is based on a combination of the absolute Supply & Demand level and the impact of a potential breach of a Safety Storage Monitor.  The basic trigger level for the alert is set by the anticipated available supplies into the network and is modified to reflect observed supply performance and depletion of storage stocks. 
A simple flowchart showing the additional Residual System Balancer actions that National Grid can take after issuing a GBA can be found at: 
http://www.nationalgrid.com/NR/rdonlyres/312CC783-1814-40EC-A1D3-79CBE318389C/10984/OTCflowchartv05270906.pdf

The basis for the Safety Storage Monitors is to ensure that sufficient gas is held in storage to support those gas consumers whose premises cannot be physically and verifiably isolated from the gas network within a reasonable time period. To achieve this all gas consumers are categorised into one of two groups, Protected by Monitor and Protected by Isolation, and the stored safety gas requirement is calculated using a traditional approach of matching supplies to demand.
A detailed methodology of the calculation process for Safety Monitors can be found here: 
http://www.nationalgrid.com/NR/rdonlyres/B4ACC5F8-A8AF-48B4-A8AA-04BE3743E1C9/13676/20067SafetyFirmGasMonitorMethodology.pdf