Have you tried yellow gripfill?

Widen the gap, gripfill it and scrape flat and then take a look at it 24 hours later (gripfill tends to shrink and expand a bit as it goes off).

Then see if it needs filling again. Once its flat and stable, prime (Zinsser Bulls Eye 1-2-3 contains latex and is pretty flexible) and paint over.

Cracks are famous for opening up above rads and fireplaces, what I would do, and you will be better off afterwards painting the whole wall afterwards.
I would open the crack all the way, about 1/2 wide, and go a couple of inches above and below it.
Then, add a bit of diluted PVA into the crack, leave to dry then fill, dont over fill it, let the filler sink in, so you still have an indent.
Sand that back, and brush the filler again with diluted PVA, keep doing this till the filler is flush with the wall.

When dry, get some of your emulsion, dilute it slightly and brush this on where you have filled, do this about 3 times, then roll the whole wall as normal.

You could also do what I mentioned above, cut the crack out etc, but put some scrim tape over the crack, then fill and again do what I just wrote.

If you use decorators caulk in the crack, there is a chance of the actual white of the caulk flashing through.

You could also scrim tape the MDF and fill like I said above.