Highly flexible, low-carbon electricity generation with gas-fired power stations with CO2 capture addresses the challenges of balancing variable renewable electricity supply in low carbon electricity systems. This detailed technical assessment of flexible CO2 capture plant operation at natural gas combined cycle power stations with post-combustion CO2 capture examines the operating strategies of capture plant by-pass and interim solvent storage. We show that solvent storage allows expanding the operating envelope of gas fired CCS power stations by +/-10%. Further we demonstrate that electricity and CO2 output can be decoupled for up to 3 hours with approx. 6000m3 of additional solvent inventory for the purpose of reducing the CO2 flow variability in downstream transportation and storage systems, mitigating potentially deleterious injection well effects. 1hr of solvent storage operation at full load can be regenerated in as fast as 2.1hrs during continuous operation of the CCS power plant by choosing a controlled steam extraction strategy from the combined cycle and thus throttling the low pressure turbine. The electricity output penalty associated with the delayed regeneration of solvent ranges from 420-450kWh/tCO2 with this strategy, which compares to 380kWh/tCO2 for immediate regeneration at full load design conditions. By deploying a novel variable speed drive integrally geared compressor model, we find that, unlike previously thought, an uncontrolled steam extraction strategy, referred as a floating steam extraction strategy, can lead to choking of the CO2 compressor during additional solvent regeneration. A pre-compression stage would be necessary under this extraction strategy to restore feasible operation of the main CO2 compressor, and makes this strategy more complex to implement. When decreasing the desorber pressure at part-load care must, therefore, be taken to respect the operating limits of the compressor. To assist with the use of rigorous plant performance data in wider electricity system models, correlations for key performance parameters of NGCC-CCS power plants at varying load, with capture by-pass and additional solvent regeneration are provided.