Hi All thanks a lot for the useful suggestions. I wasn't sure how much of a bodge Flashband was supposed to be, nor how functional.
I am now armed with some flashband and primer-type sticky stuff. When it stops p*ssing down for more than a few minutes I will have a go at drying things off and applying all of this.
I've also spotted that the gutter above needs cleaning, despite me doing it only six months or so ago. That won't help matters...
Thanks again Jon N