Fine grade emery might be ok for removing surface rust but it aint gonna get rid of the pitting. For that I think you'll probably need to take the hob plate to a machinist and get it skimmed. Then you finish it off nicely with some of this..
www.dmpb.co.uk/hpr.htm
That's all very well if you have know a friend of a friend who can do it on the cheap otherwise it might be cheaper & easier just to get a new replacement hob plate (if they are still available).
AGA suggest this:
"The hob surface can be restored reasonably easily in situ by using grinding paste and an oilstone or suitably shaped flat piece of steel. But, what ever technique you use, donââ‚Âà ‚¬ÃƒÂ¢Ã¢â‚¬Å¾Ã‚¢t get the paste on the enamel surface!"
Which is just great if you've got very big biceps and several days spare to grind out any significant pitting
.