How to make persistent hairline crack disappear for good?

[Flipjango]

We have a hairline crack where the extension meets the old house that will not die. The extension build is fine, tied in, all lovely (I watched it being built so I'm not unhappy with anything the builders did). The crack is tiny - maybe 1mm wide and vertical (pretty sure it's where a sheet of plasterboard meets the old plastered wall) but due to its location in the middle of the kitchen wall it's really obvious. I've raked it out, taped it with scrim tape and refilled it twice now but it just keeps coming back. Because the wall is completely flat and white and the crack is tiny, trying to use something like Toupret flexifill doesn't really work as you can see the repair - it's too hard to get that stuff perfect and it doesn't seem like it can be sanded. I'm probably doing something wrong with it - maybe someone has a tip that will help me get it right if flexifill is the solution. Please help me make this crack go away for good!!!

 

[Jurassicspark]

Open up the crack with a carpet knife and fill with any flexible filler or even caulk.

 

[Jurassicspark]

Something like this.

FC1 - The Ultimate All-In-One Filler & Caulk

The ultimate all-in-one filler and caulk. FC1 bonds and seals gaps between a variety of materials including wood, metal, brick, stone, plaster and plastics.The super-smooth finish doesn't require any sanding, and it's quick-drying - it can be painted or papered over in as little as one hour...

www.toolstation.com