is your dpc still under guarantee, if so get them back round to sort it out.
you can cover over with boards and plaster but that is all you are doing, it will still be there effecting behind the board and eventually appear again.
As the saying goes a stitch in time.......
Hi, i take it this problem is on the ground floor? when the dpc was replaced were the cavities thoroughly cleaned? a bridged cavity will cause damp problems regardless of age. Also have you checked your dpm hasn't failed, if applicable! sometimes though damp can come in from failed flashings etc... from above, you need to check these before you carry out any finishes! Hope this helps.
do not just plaster or board over wet existing plaster especially if caused by damp. out of sight is not out of mind. you need to sort out the damp problem, find the cause and fix it before you go covering it up. otherwise it will come back and bite you on the ar*e and the damp will only come through again anyway.
however, once its sorted and completely dried out you can skim or if the damage is minimal then a light sanding followed by a damp stain block such as aquaseal then paint.