ferroli domina 80E boiler shutting down?

Hi

I have an on going and worsening problem, my hot water runs cold and pilot light goes out after 1-10 minutes, it seems to be getting worse. If I run the hot tap at just maybe half a turn of tap it seems to run for longer.

I have had Brit Gas in several times, they have changed sensor and pressure gaige and have now fitted a Hydroflow saying it must be limescale build up that is heeting pipes up and backing water up so boiler thinks over heating. But I have seen no improvement if anything worse, it has only been fitted a day though.

Alsoo if I getthrough one shower OK and someone goes in shortly after they have no chance of more than a minute of hot water

Any thoughts, tips advice truely welcomed