I'm afraid movement in GRP hulls is a fact of life. I'm not a chemical engineer, so I can't say for sure why the material tends to move and creep, but my first instincts are that it never truly cures, it just goes from a liquid state to a harder state, but it's still in a state of flux, influenced by fluctuating temperatures and stresses in the material. You can manipulate glass laminates using heat, but serious warpage is difficut to correct without serious surgery sometimes. If you purchase a boat secondhand, or don't build it right from the start, and the moulding are allowed to bow out of shape, then you may have to cut the hull, and rejoin. This can prove troublesome if the hull has a lot of detail moulded in, as bits tend to be wiped out in the process, requiring the detail to be reapplied.
Some models are known for being a problem from newe.g.the Engel Typhoon has a warp in the top half of the moulding which requires correction by the builder. Warpage can occur though inadequate thickness of laminate, poor lay-up, incorrect storage, a bad tool, extremes of temperature and unequal stresses placed on the hull.
However I've yet to see a hull that can't be salvaged, it just depends on your level of skill and patience.
There are two S-Class kits on the market. The MBD S-class in the slightly odd scale of 1/38th, which was originally produced by John Darnell. £245
http://www.modelsbydesign.co.uk/model_boats.aspxThen there is the slightly larger 1/32nd scale OTW kit. This benefits from photoetched deck and other details like rivetted conning tower, brass metal fittings etc. £695
http://www.otwdesigns.com/submarines/SClass/SClass.htmlBoth kits are moulded in polyester GRP, so they will move unless you take steps to ensure they are braced/pegged in some way. Epoxy glass hulls are less prone to this kind of movement, but they can still move, especially if they're left out in direct sunlight on a hot day. Both build into nice models at the hands of a skilled modeller.