Sparklers... being on the edge of a flood plain will likely mean that there is a high water table. Meaning that, if you dig a hole (including foundations), then it will fill with water.
The volume of water falling on next door's ground is still the same as before. What has changed is the ground's capacity for soaking it up.
The building is taking up a lot of the area where water used to soak in.
The rain is now running off the roof and carried into soakaways. The question is... are the soakaways enough?
That comes under "Building Regs". You might try and get Building control interested. No property should ever legally be able to discharge their water to a neighbour.
Be prepared for "buck-passing" by the Authorities. It's a Building Regs matter, but also should concern The Environment Agency.