My Worcester 24i junior boiler keeps having a fault?

I have a worcester 24i junior boiler, and lately i keep getting the fault light flashing. I have gone through the manual, and doesnt really shed any light on what is wrong. The first time it happened I emptied the syphon and a load of aluminium oxide dropped out. I restarted the boiler and it worked fine. Since it has gone cold the last few weeks it has happened again, so I repeated what I did last time, and again it was fine. Now, it keeps stopping and am finding myself emptying the syphon every night.

Any advice?

Thanks