If your hot water is working and not your radiators then the problem is to do with your boiler and not your cylinder, it could be that your timer is off or the gas valve is not lifting in your boiler, Thats is what you pay a Gas Engineers to work out and fix.
Get a qualified Gas Safe Engineer
p.s. Take into consideration that not all qualified engineers know what they are doing. LOL
Welcome to the world of cowboys, very hard to find a good gas engineer, you might have to search hard or find recomendations.
Good Luck!!
Taz®, April 2010