Tip

Should you have a house with a valley roof and thus a drainpipe running through the middle of it, and should it be a bit damp for one of many candidate reasons, the solution of "making it all go away" by hiding it behind plasterboard is not  a good one.

If you're feeling especially dim, stuff the void with loft insulation (like, why?) so that it can really hold the damp in.

Spent most of the day discussing solutions with plumber & roofer and cutting my way through the plasterboarding and into the void.  Thankfully no walled-in bodies, though the space was big enough.  

Not sure what the red is.  Wallpaper, blood, Martian weed, or some interestingly colourful virus about to wipe out humanity?

If the latter,... err,.. sorry.