Hi Micheal,
I am nearing completion of a damp repair to my van, sorry dont know about the ceiling panel fitting, as mine, was to the walls and I don't want to be the bearer of bad news.But I would assume it's near enough the same principle.
My walls started, as yours with damp patches and after removing the wall board it uncovered a can of worms, which then has to be rectified.
Hopefully you have caught yours earlier than I did.
Have a look at the photo's in Technical under the heading Hidden Screws? do a forum search from the blue box in the top right hand corner (sorry don't know how to do links).Don't take them at face value as it's not that hard to correct but very time consuming.
If you require further info please ask.
On the bright side, if you are a confident DIYer then I would say go for it but if you have any little doubt's then give it over to an expert (with your bank balance!!!!!)
Kev