Dormer Window

I have a dormer window on my roof which is glazed on 3 sides, however the tenor and mortice joint has been broken (and rotted) at the bottom corner angle.
I plan on filling the rotten wood with epoxy (recomendations wanted) and the battening the sides and coverring with Upvc Cladding.
Am I better boarding prior to battening or better just battening over the top.
Any recomendadtion on external wood filler epoxy to use and also should I use a membrane?

If the dormer has a pitched tile roof, the frame structure is holding that up as well as supporting the window frame(s) & the glass! Unless you just tarting the place up to sell, I would be inclined to repair the dormer structure properly rather than bodge it with epoxy filler & cover it up with UPVC; what your proposing will only ever be a very temporary repair!