Are any running outlets available to use with a 112mm half-round UPVC
gutter (yet to be purchased) with the existing 53mm cast iron downpipe
already fixed to my wall? It's just that i'm replacing two
cream-crackered wooden gutters on single-storey extensions, one of
which already has a standard 68mm UPVC downpipe (easy) and the other
has this old metal one (which is otherwise fine and I really want to
use it if possible).
Been to Screwfix and purchased the end stoppers for both gutter
lengths, all the clips and screws I will require, and a running outlet
for the 68mm pipe. Had a look in B&Q and they sell "Miniline" 53mm
plastic downpipe, but no running outlets for the 112mm gutter/53mm
downpipe combination I want.
(Flow rates won't be an issue as both roof sections are very small).
Could I just butcher a hole in a length of 112mm gutter above the metal
pipe and seal it with Screwfix's "Sticks like Dung" or similar ;-) Any
further ideas from the floor?

Thanks in advance.