how can i fix my ,worcester heat slave combi boiler?

This is what happens when i turn on a hot tap, first the fan starts and after a short while the pilot light ignites but then nothing else. Not sure wether to checkout the Gas Valve or maybe find out where or if there is a Flow switch in the circuit. The Central heating switch doesnt bring it on either, Ive topped up the water pressure ,it now reads just over 1.5 bar ,
Any ideas anyone? would appreciate any help!