You will probably, but not definitely, find that the pipes are connected to one another using a "solvent cement" so you may need to be a bit inventive with sleeves etc.
Hmm, that sounds sticky... Might I be better off just replacing the bottom section of downpipe entirely and putting the diverter in that where I need it? Or is that easier said than done and would it also likely require messing about with cements?
Perfect, thank you! I was putting in many weird search terms (junction/sleeve etc) but this is exactly what I want.