Vinegar and Water: Fill a microwave-safe bowl with one cup of water and two tablespoons of white vinegar. Heat it on high for five minutes, then let it sit for a few minutes before wiping the interior ...

If you don't see any steam, give it another minute (my steam is visible on the cupboard above the microwave, which is how I knew it was working). I let it sit with the door closed for five minutes.

Panasonic's CS89 combination microwave (or, 4-in-1 combination steam oven NN-CS89LBBPQ as you might see it listed) costs £469.99 in the UK and is available from all the usual electrical retailers.