Water runs down outside of the downpipe.
If you have water leaking from the top of your downpipe, it is probably not the downpipe that’s blocked.
Usually, when you see water gushing from the top of the downpipe or swan neck, it is because of a drain block that the downpipe feeds into.
Your drain and possibly the base of the downpipe will need to be unblocked.
An excellent way to check is after rain, tap the downpipe, and if it sounds solid, it’s probably blocked at the base.