Your boiler is simply letting you know that it is pre-warming the water so you get hot water at your taps more quickly.

DIY fix possible

No action is required as this is a normal operating function and not a fault.If you wish to stop the flashing, you can turn the 'Preheat' or 'DHW' selector dial to the off position.Monitor the boiler for any alternative lights that stay solid, as this would indicate a genuine issue.