Thanks to all of you who have posted a reply so far. The concensus is pretty overwhelming and I'm convinced - I'm dropping the idea of sheathing! I guess I had only read the success stories.
A number of you raised questions and I'll try to answer those. Firstly the question of leaving the boat in the water. The issue here is that the boat lies on the south coast of Norway where there is a danger of ice during the winter. Next year will be even worse due to the fact that we are moving to Oslo where it almost always freezes in the marinas. So basically I have to have the boat out of the water from october to april. The only bright side is that the temperature is low.
Another question was about caulkin. My folkboat is clinker built so I will have to replace all the fasteners to stop water seeping in between the planking. There are some sizeable gaps in the planks themselves but they close up well. But between the planks there is a constant seeping in some places.
One of you asked if the broken ribs where near the mast foot and therefore possibly the cause of the wekness in that area. The answer to that one is no, they are not related, the broken ribs are further back.
I think I can summarise the advice as follows: Make plans to get on with it: New ribs alongside the broken ones, strengthening around the mast foot, replacement of badly split planking and redo most of the fastening. Agree?
So my follow-up question is ... are there any good books on how to do the above? I'm competent with most things but it's great to study the subject before you start and try and avoid mistakes.
Once again I appreciate all the contributions (and the humour!)