Thinking more about your problem it could be a problem with the cold water supply to the shower. When you first turn it on the hot water will be cold. As it runs it will get hotter until it exceeds the temperature set on the control knob. At this point the control will start to add cold water to keep the temperature below the set level; if there is no cold water flow the control will turn down (and eventually off) the hot supply to keep the temperature down. When you turn the control to "hotter", hot water will start to flow again until the temperature reaches the set limit and then stop.
I suggest you look for a stop-valve somewhere in the cold feed - it should come directly from your cold-water header tank and SHOULD have an isolator in it somewhere; if someone used a stop-cock instead of a gate-valve there it could have got stuck. If you don't know about these things, stop-cocks look like ordinary brass taps, gate valves usually have a red circular knob to turn them on and off.