Gas moves through National Grid's transmission system at 25mph, which means that additional supplies cannot be delivered instantly when demand increases. So if gas is needed in Newlyn in Cornwall - the most distant location west currently supplied - and the gas has been landed in Scotland, it would take around 23 hours to reach its destination. It cannot instantly arrive at the location of increased demand so the gas transmission system has to be kept constantly in balance. National Grid ensures a balanced system by buying and selling gas and using stored gas to ensure that in most normal circumstances, demand can be met.
Usually the system is balanced by gas bought on the open market through the 'On the Day Commodity Market (OCM)' set up as part of the Network Code - the rules that govern the supply of gas. National Grid also has gas in store (the so called Top-up and Operating Margins gas) to ensure the system is balanced and that supplies are maintained.