Gas's contribution to the UK's electricity supply is very reliable. Under normal circumstances, and given a sufficient supply of fuel, 90% of the UK's gas-fired capacity can be assumed to be available to help meet peak demand for electricity. Fossil fuels like gas inherently store energy which can be converted into electricity on demand by burning them in a power station. So given a secure supply of fuel, gas-fired power stations are capable of rapidly increasing and decreasing their output to meet demand.
These are the reliability challenges facing gas. Read about the possible solutions.