Unfortunately AlanO - I think that with current weather forecasting technology - going beyond the standard 3-5 day outlook is never very accurate and all predictions I have seen that far ahead seem very vague.
Having said that, there are people and organisations that attempt to do what you are asking, but it's seldom free advice unfortunately - For a free service I would heartily recommend googling for Terry Scholey - his monthly predictions have been pretty bang on for the last year or so.