riva 24 model number1306407883?

Probably not. The riva uses a magnetic flow switch positioned slightly right of centre of the water heat exchanger behind the control panel. It is a small plastic switch with a tiny red neon light on the top surface that lights up on demand for hot water. If the neon isn't coming on it's probably the flow switch. Not really a diy job....
If the neon is coming on then it's a control fault further in.
You will probably need to get a service engineer. Most installation plumbers would not be interested in this kind of repair.

T.H.S
April 2007