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?
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.