No hot water on Ideal Mini HE boiler?

Hi,

i have an Ideal Mini HE boiler and the there is no hot water unless the heating is on at the same time.
When I try to draw hot water, I can hear relays opening on the PCBs and the boiler tries to light but eventually comes up with an error indicating that there is 'no water or absence of flow'
There is pressure in the system according the the pressure gauge.
If the heating is on, then there is hot water, as I guess the boiler is already lit.