Buy the foam round lagging and cut V shapes in the side that has the slot
This may need several goes so start small first
Bend them around the pipes and use insulating tape to hold them in place
You can also get a sort of felt lagging bandage
Insulate both pipes
If you have left over loft insulation then construct a box around the area and stuff in insulation, use a bit of hardboard or ply to cover it
the stiff plastic foam lagging will bend into a moderate curve. Place it (in your case) so the slit is facing downward, and fasten it with plenty of tape so the slit can't gape. I've used it on tighter curves than that.
If those pipes are in an unheated area (not a habitable room) you should use the "bylaw" grade, which is thicker than the "economy" grade
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