It's quite common for Aga range cookers to have a bit of rust in their ovens, on the hotplates or on the hotplate expansion rings. It normally happens when the Aga range cooker is off for awhile and collects a little moisture or condensation and nothing really to worry about. You can avoid it by leaving air gaps and oiling hotplates. See here for more info. Here is what to do to sort it if it's already happened!
That said if your rust problem is severe, especially if you rarely or never turn it off, then there could be other causes to check and resolve, lets run through a few...
- Power flues can cause problems. Using a fan to force the air flow is fine until the fan doesn't work - make sure it does or steam from cooking and, much worse, combustion gases are not swiftly removed.
- Electric range cookers often have a type of power flue. It's not necessary for combustion gases of course so is a smaller brass pipe with a fan at the end that sucks cooking smell and steam out of the ovens. Sometimes kitchen fitters get carried away and make this a quarter mile long with myriad corners and gradient changes - it won't cope! Make sure at the very least the fan is fitted properly (no gaps around it) and working to create a suction from your Aga range cooker. If not then steam and moisture simply rattle around in the cooker and rust everything in sight!
- Balanced flues go straight out the back of the cooker. Check the cowl and for obstructions as they are more sensitive then the traditional vertical flues.
- On your traditional flue check the cap!