In reply to Postmanpat:
> Although I pretty much know my dates it's not impossible that plans will change. Do prices tend to go up nearer the time in the Jan-April season or can I afford to wait?
The Brittany ferry crossings (Portsmouth/Plymouth to St. Malo etc.) go up quite a lot if you don't book very early (or at least the Bilbao ferry did when I was checking it last year), but the Dover to Calais P&O crossing doesn't seem to change much.