I never descaled ours... Could you not just use a descaler sachet, mix it in a jug and then pour the correct amount into the steriliser before putting it on a cycle? Then rinse it out plenty of times before using with your bottles. After that, can you use filtered water in it to stop the limescale build up?
If you need to sterilise bottles in the meantime then put them into a big, lidded pan with just enough water so that it doesn't boil dry, heat it on the cooker and keep it there for at least ten mins. I hope I'm not offending your intelligence by telling you how to sterilise without a steriliser, it's just that some people have just never thought to do that.
I get sachets of descaler from the baby section at boots (in a plainsh white box near the steriliser tablets etc, our boots is quite a big one) then follow the instructions on that box. It's something like put in 200ml water, put steriliser on for 5 mins (set timer) then pull out plug and add contents of sachet. Leave for 20m to work & cool, rinse then run empty normal cycle. (On DS2 in v hard water area. Do this monthly, does it show?)
Btw baby steriliser descaler is lactic aid (ie milk- related), normal kettle stuff we use (killrock) is formic acid. Prob no reason not to use killrock/limelite etc with method above if you have it in house & it's urgent just run a couple of empty normal cycles before putting in bottles.