Put something like this where the existing diverter is:-
http://www.diy.com/departments/floplast-black-pvcu-downpipe-socket-dia50mm/81066_BQ.prd
Install the diverter higher up (by cutting a section out of the existing pipe).
Remember: Measure once, buy twice.
Perfect, thank you! I was putting in many weird search terms (junction/sleeve etc) but this is exactly what I want.
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.
@owl started
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Put something like this where the existing diverter is:-
http://www.diy.com/departments/floplast-black-pvcu-downpipe-socket-dia50mm/81066_BQ.prd
Install the diverter higher up (by cutting a section out of the existing pipe).
Remember: Measure once, buy twice.