Togo's answer is correct. There really isn't likely to be any fault and is almost certainly due to heat being transferred from the food container to lining. Moving the appliance away from the socket won't make any difference.
If you Google at what's inside a magnetron (the component that makes the microwave energy) they are remarkably simple components in terms of what's inside, with so little to be made incorrectly.
Furthermore, it's a 'Sharp' who are pioneers of M/W ovens, who have been making them for over 60 years. My mother in law bought one from Asda for £75 a few months ago. She's bordering on being paranoid expecting every single piece of technology to either be faulty or kill her. I'm taking her for a hospital appointment tomorrow. If she's annoying, I'll ask her if hers is getting too hot. Otherwise, I will heat something up for five minutes and see if it too gets hot, as I am faily sure it's the same 20 litre model as yours.