Heating but no Hot Water?

Hi, I have a LP522 timer on my heating system, and have just moved in. The previous owners didn't leave any manuals so I'm a bit stumped. For the first few days I was just using the water heating, on the timer and sometimes at random, and it was working fine. Then I decided it was a good idea to check the heating so I tested it and it worked fine. However, since then, whenever I turn on the water via the timer switch, only the radiators come on and I have no hot water, even when I turn the radiator thermostat down as low as it can go. I am definitely using the hot water switch and not the heating switch, it is the same switch that was working fine before I tested the heating! Any ideas anyone? Thank you!