You might re-consider using 2 covers. One of the most important issues in storing boats, especially those with a wood/fiberglass construction, is VENTILATION...moist air at a certain range of temperature, promotes ROT...you want more than anything to keep it dry inside and allow air to CIRCULATE...that's why I say one cover, put moisture absorbers inside and open it up and air it out a few times over the winter. The UK is a damp climate like Long Island NY where I live and I'm always trying to air the boat out on a dry sunny day in the winter when I can open up the cover. Two covers =mildew and excessive moisture.....
http://www.rotdoctor.com/glass/GLmain.htmlread this, very informative, especially with regards to storage.