Heating works but no hot water?

I have an Ideal Icos HE15 Heat Only boiler which is about 2 years old.

Last week the hot water stopped working. The central heating ignites and works but when it comes to the hot water, the programmer says its on but there's not so much as a sound coming from the boiler.

On the Ideal website it says it can't be a boiler problem because the boiler can't differentiate between heating and hot water modes.

Luckily the system has a back up immersion which works perfectly which makes me think its not a problem with the storage tank either.

Has anyone any idea what the problem may be if it turns out not to be the programmer?

Thanks in advance.