Your ad is fairly old but if you are still looking, I may be able to help
I am looking to sell our one-eleventh share on NB Freedom (There is a share for sale on the ads page, but that one is not ours - I have only just found this site!)
Freedom is a 60ft trad narrowboat, which fits your style spec and the way weeks are allocated on Freedom sounds like it might be exactly what you are looking for.
There is an annual draw for choosing order. Weeks are chosen one at a time, first one week in draw order, then one week in reverse order and so on, until the weeks are all allocated.
We get a minimum of 3-4 weeks a year, depending on draw order. There is no restriction on when you take your weeks, which can be between the beginning of March and the end of October, would fit nicely with your requirement for require late autumn winter and very early spring use.