Mark
The best thing I've found for the gap between the tip of the jacket and your neck is a Buff (
http://www.buff.es/default.cfm?idioma=3) available for about £12 from most bike, mountain or cycle shops. Warm enough in winter but also comfortable enough to wear to stop insects in summer. I have half a dozen and wear one every day.
Not at all waterproof, but I don't think you'll get enough water in at the neck for that to matter. The small amount that gets into that gap will be soaked up by the Buff and heat from your neck gets rid of it quickly. I rode for 150 rainsoaked miles in my summer jacket with a Buff on the only really rainy day of last summer. When I stopped at the end there was a bit of dampness in the Buff and none down the inside of my jacket. My wallet on the other hand, which I put in the outside pocket of a jacket with a zip in waterproof lining, was wet enough for me to have to dry my money out with kitchen towel and a radiator