Hi, to confirm are you talking about a gas hob?
This repair would be around £60-80, Zanussi hobs are rather easy to repair, as this is a gas appliance it will however require a Gas Safe engineer to do the repair.
It is most likely the jets which sit under the burners have become blocked, they unscrew in a matter of seconds and can be cleaned or replaced, a complete set can be found here for under £20.
Service Force are the recommended repair firm, however they do charge around £60 call out so your looking at the top end of the repair cost.
Please let me know if you require anything clarified.