I have a glow worm fuel saver F?

I have a glow worm fuel saver f. I have hot water but no central heating. When you turn the timer on I can here it click on but nothing happens. I have checked the thermostat and everything seems to be working ok. Could it be the thermocouple and if so, how much is it going to cost to replace?
Please can anyone help? It's starting to get a bit cold!