You need to know the model number 1st which is indicated in your Cooker Instruction Book or your Service leaflet. Or it may also be labelled on the back or bottom of your cooker. If this is not available the part serial number of the part is stamped onto the bottom of the hob ring itself. However i would contact a creda spares supplier to ensure they can use this information eg. http://www.appliancespartsdirect.com/
Your cooker model number if white like mine may be 48331 and need a 1500w / 850w twin Hotplate replacement part if power supply is 240v ac. However there are numerous models with different part requirements and your may be different.
To actually gain access to the hob is the same process to removing the glass top of the cooker outlined in a previous answer on this site.
Chris Mulhall, November 2009