Well you seem to have researched the job just take care a bouncing router is deadly as I said before check Mighton site they supply all types of seals for windows and the cutters along with some useful vids and drawings nothing to loose
Tips like this from someone like yourself with experience specific to my task are like gold dust. So I looked at the Mighton site. Although there are some things about casements there (all my windows are casements), it seems mainly about sashes. The weatherstrip seal I'm favouring is a bulb seal, as there is (very conveniently) a shallow groove, maybe 8 mm wide, running all around the insides of the FIXED frames of my windows: hopefully a bulb of suitable dimensions, attached to the frames of the CASEMENT frames will be able to fit quite snugly in there, helping block out both cold and noise. I'll have a deeper look at the Mighton site though.
Could I ask you to try and identify the type of bit here: ... at 5 min. 35 s? This guy describes this bit (used for rabbets/rebates) as a '3/4" straight bit with a bearing on it... top-mounted bearing'. But to me, in that vid, it doesn't look particularly straight: I've got the impression that straight bits are not always that effective, and that various non-straight bit types can help clear away the shavings more efficiently, for example.