Problem with hot water - any help gratefully received!

[emmaelemer]

I have a Marathon boiler - central heating is working fine, but the hot water isn't
I can get hot water if I manually switch the water heater on...but not if the timer on the boiler is on (even when the timer is put on constant).

Please excuse my ignorance and lack of terminology - hopefully someone will know what I mean!

Thank you for any help...struggling here!