Thank you all for the help - it is remarkable how much knowledge is available here. Zeus especially - that was quite a post.

I did check with my building inspector, and he made some of the points mentioned here, including recommending removing the shingles. He was indifferent to whether we put new sheathing over the old, or took off the old. His only concern was that if it is an older house (which it's not), be careful of overtaxing the rafters.

We currently have a ridge vent running the length of the roof. Have these been improved on in the last 25 years, that we should think of replacing it?

We also have many skylights - so I was thinking that we should do something special around them. Up to now there have been some small manageable leaks from them, but I don't want to tempt fate.

Interestingly, he said if we remove/replace the sheathing or add a new layer, we will need a building permit. I suppose it's not a big deal and hopefully we won't be re-assessed.