There should be nothing 'down to you' ... just make sure that they do thoroughly check the connection to your complete satisfaction - it would be worth asking for a replacement laptop if they are unable to offer an otherwise satisfactory explanation.
You could mention that you have obtained advice from this Forum!
It's a fault often found with DELL laptops usually caused by the soldered fitting coming adrift, In my opinion it will get worse & you will get intermittent connection, then eventually none at all.
Ask for replacement, I would not have any other option, if I had to stay in the shop all day. The power in socket is often soldered directly on the the ma mainboard. Very expensive to put right when out of warranty, if worth it.