Worcester Greenline CDi boiler - ECO button will not stay lit

Hi
As the title says the ECO button on my Worcester Greenline CDi lights when pressed for a few seconds, but goes out sometime later. I've not sat and watched or timed how long it stays lit for, but generally if I check within a few hours, it will have gone out. I think I've only once stay lit for more than a day.

Any ideas what to check?

Thanks

Ian