stuck with my Ariston boiler and its getting colder by the day?

Hello I have an Ariston A/27 MFF1. I have just tried the heater first time this year and found that it cuts out when the hot water is run and insufficient water light comes on. The pressure gauge is unaffected when this happens .I have drained the system twice and flushed it once.any ideas please.

Pete
I did remember I had a new flowswitch and pcb fitted in june. I got the installers out and they said it is a blockage in one of the boiler components which is unfortunately not covered by my homecare agreement because its a problem with the water rather than a component fault (ye right mate) anyway they offered to perform something called a sonic clean of all components for a cost of £200+ with no guarantee that this will cure my boiler problem. does anybody know of a cheaper and more effective way to clear the blockage in my boiler please.