On all Aga range cookers it's a good idea to allow air to circulate under the lids while the cooker is off. If not you can get a little condensation that can rust the hotplate surface the the iron expansion rings around them. When you cooker has cooled completely use a little kitchen roll with olive oil it and wipe a thin coating across the hotplate and expansion rings to protect it. Now find an old pencil, snap it in half and pop each under the front of each lid to raise it very slightly. This will let air circulate without looking ugly!
If however you are new to Blake & Bull (welcome!) then you'll probably want to know what to do about it once it has already happened. It all depends where and how bad it is. All the below goes for hot plates rings too. Remove them before tackling the hotplates or using any of the techniques below and replace them if really bad.
For lightly rusted hotplates... Use a dry wire wool (washing up wire ball) and give the hotplate a damn good scrub, you'll turn the rust to dust! Once done use a vacuum to suck the dust up. This is better done cold but can be done hot if you are VERY careful about where you hold the wire wool. When hoovering the dust up use the metal bit of the hose, plastic will melt ;) It's best to avoid touching the enamel around the hotplates so be careful. If leaving cooker off now is the time to oil the hotplate to avoid it happening again!
For heavier rust... Use a small tough wire brush (get them at Wickes/B&Q etc) and really go at the hotplate. Be careful to avoid the enamel - wire brushes will damage it. Once done again vacuum up the mess!
For REALLY thick rust (like the one below)... Buy a wire brush attachment for your power drill. Use a variable speed drill for control and scour that rust off the hotplate. You need to be VERY careful. Use eye protection, do not touch the enamel and just go slow. This method works well but is the riskiest as enamel really doesn't like the power drill/wire brush combo!
If you have rust inside your ovens then the only thing to recommend really is the wire brush. Power drills don't fit inside the ovens very well and the wire balls aren't really man enough for the job!
Buy a big solid wire brush and patiently get in every corner. You'll need the ovens cold for any kind of access. Once done again vacuum out the dust/mess. Aga range cooker ovens rarely look great so don't worry too much, they aren't on show like your hotplates!
If you have severe rust on hotplate and inside your ovens there may be something a little weird going on - don't miss this post for possible explanation.