How to descale a thermos?

[Whenisitmysleepytime]

I have been using a thermos for a supply of hot water through the night for dd's formula. She's now done with night feeds so I can put it away.
But it has huge amounts of limescale in it

I've put vinegar in it but it's not shifting yet.
Any ideas to speed it up?

Use lemon juice it is far better than vinegar and leave it to soak overnight. Also won't leave a smell like vinegar can.