My plasterboard has developed vertical cracks in it.

Vertical cracks normally indicate plaster board. As Brian has said,was scrim tape used on the joints of the boards before skimming or are they just dry-lined? Saying that though,even using scrim on plasterboard joints is no guarantee that cracks won't develop. Even the slightest bit of movement in the wall or boards can open them up,as can differing temperatures/moisture etc... in the rooms. I don't think it's anything to worry about,they can easily be repaired with some filler,sanded down and re-decorated. You might be lucky when you've done this that you won't have to do it again but don't be surprised if you do. Carl.

If its plaster board, did they scrim the joints ?

I believe that these cracks are just due to settling and shrinkage and that it is nothing serious to worry about. To resolve the problem, there are a few options. Thses would be to: 1. rake out the plaster back to the block wall (cut a section of the board out) and to fill this with drylining adhesive or expanding filler, and then fill or plaster over before painting. 2. you could fibretape over the crack and plaster over this. You would need to find a good plasterer to be able to patch the plasterwork well. 3. A good decorater could cover with lining paper which would cover the cracks well. Cracks are very common in barns so dont worry. As long as you cannot slide a £1 coin into the crack then they should be harmless.