What you need is the plastic stick on gutter, it's about a foot longer than the door, look on Ebay.
Thanks for the suggestion. Do you mean the Fiamma stick-on gutter?
Suggestions in forum posts seem to be divided between external guttering (like the Fiamma), and some kind of internal rubber seal. I intend to try the internal seal first - hence my question about the P-shaped seal, but not sure which one to go for.
Some say that the Fiamma guttering doesn't stick very well (the ends curl away from the body).
See the review on this page
Others say that the adhesive sticks too well, and marks the body should you wish to remove it - something I want to avoid on my new vehicle!
But if the internal seal doesn't work, I will try the Fiamma guttering.