Treat the rot, apply wood hardener, fill in gaps with 2 part wood filler, …but I’m left with a largish golfball size hole right at the edge where it meets the brickwork…best fill in with cement mortar? …rain has been running the patio edge…normally would run off the cill but it’s missing there and goes into the hole and into the house!
Need to finish with good quality undercoat/primer and paint
It is mainly for masonry though, not sure it would work on wood,
Like silicone on steroids?
Heres how the hole looks hoovered out. Can’t be sure but looks like damp proof membrane? Applied the wood hardener z waiting for it to dry then then wood filler, it’s the big hole that has me head scratching.
Yes, I would use repair care it’s not cheap but excellent for jobs like that. This was a badly rotten external door that I fixed with it. It’s lovely to work with and when dry you can chisel, shape and sand it. I would definitely use it (instead of a normal 2 part wood filler like Ronseal) on your job.
Other option would be to square it off and splice a new piece of wood in, then fill whatever’s left with either above sughestion…
That is the DPC but it looks a bit short to me! - it should ideally rise 150-200mm up between the brick and the frame.
I would put a lot of silicone in that gap. Then as Jonty suggested cut in a piece of timber and fill over