if the A and B lights on the boiler are flashing alternately this means that the system pressure is too low and you may see the pressure gauge indicating that it is below 1 bar and outside the green area.
To re-pressurise the system you will need to follow these steps:

Find a filling loop (metal braded flexible hose) on the underside or underneath the boiler with one or two tap connections at either end of the filling loop.
Open the tap or taps a quarter of a turn which will allow water to refill the system (which you will be able to hear as it happens). You will then see the pressure gauge rise back up into the green area between 1 and 1.5 bar pressure.
Once the pressure gauge is in the green area, turn the taps back to the off position and the water will stop filling the system. The lights should now stop flashing and the boiler will now operate. If the lights still flash, top up the system a little more but not beyond the green area.
If you overfill the system and the gauge is outside the green area, just bleed water from a radiator until the pressure on the gauge goes back down into the green area.

For further information on this topic please take a look at our article Dealing with boiler pressure.