no heat to radiators unless hot water on?

There's a valve - it directs water to the rads or to the hot water system and sometimes to both - sometimes they get stuck. Diverter Valve / Mid position Valve...

http://www.centralheating-diy.co.uk/images/3_port.jpg

Your boiler engineer will soon sort it out for you...

Good luck...

Peccavi
January 2010