hairline crack in the middle of windowsill

Did you mean to upload a photo?

What is the construction of the wall, double-leaf cavity or breeze-block...? Is there a crack visible on the sill or could the damp be just condensation?

Is the crack on the exterior as well as the interior?

Were there any recent modifications to the building or do you think that the crack has been there for a long time?

If it's an old crack and the building has settled, I'd fill it with a good quality exterior filler such as Toupret masonry-repair-filler or Sika Sikaflex EBT, but I'd open the crack up first with a screw-driver so that it's exposed and the filler can fully fill it. Then repaint the wall with a good masonry paint to keep the rain off it.

Bassy, that's a mess alright and it's bringing in dampness and causing internal humidity issues, I can see. 😣

It looks like it might be a combination of that cracked sill and the seal at the bottom lip of the window which is the cause, but the right exterior window reveal was repaired in the past and it looks like it has cracked since. That reveal problem may have happened due to spalling (water ingress into the concrete which causes the steel reinforcement to expand outwards). Really it should be looked at by a competent person/engineer, but I believe that most of the water ingress can be temporally eliminated by applying a silicon sealant to the areas.

The sealant has to be application specific such as the below - no point in doing this with bathroom silicon.

The two cracks (one in the sill and the other in the right reveal) need to be raked out so that the paint is removed and the concrete exposed. The joints also need to be fully dry. Then apply the silicon pushing it well into the cracks, then smooth off and leave it to set. The very bottom of the window where it meets the sill also needs to be done, except it just needs to be cleaned well with a general cleaner and then dried off well. Then apply a good bead of silicon along the whole joint, especially up to the walls/reveals. Again, these are only a temporary fixes, the overall problem still needs to be looked at as there could be other issues there which we just can't see.

Tell me, do you have the tools and ability to do this?

See, it all depends on whether that sill needs to be replaced or not and whether that crack to the right-exterior is spalling or not. Spalling can be fixed, but it's messy as the reveal will need to be pulled off and then chipped back, the steel treated, and then re-formed/concreted.

The main thing is to stop the ingress of water into your home. Honestly, I'd go with what I recommend above (see if others have additional ideas also) and paint over it if it improves matters. If there is no flexing of the joints then it should last 15 years or more before needing repair.

Option 2 would be to get a builder in (even a lad doing a local job) and have him give an opinion.

Hopefully this helps!

I was initially thinking to use the silicon for both to be honest, but now that we know that the reveal is solid then if you want to now try repair the sill crack then use the Toupret or another good quality concrete repair filler. I only mentioned the Toupret as it was recommend on here by another boards user, but there are others out there which are probably fine too.

If you're going to use a filler on the crack then use a standard weatherproof sealant on the window-frame. Examples here:

It could be 50 or more years old, that construction. Not sure how mass-concrete builders handled DPC's back then other than bitumen and the like. Also looking at the external reveals I think that there is an indication that there was a refit at some point in the past. If the internal sill comes off, we might see what's going on at the sill level.