do you have any other rain water pipework, we had only one in 1904 semi that went into a drain and into the main sewage.
However in our current bungalow all the gutters go into soakaways, I had to replace the one in the front.
At that time the Neighbour spoke to us as he thought the gutters went into the sewage pipes running down our property. On
investigation I found that the sewage did BUT all his gutters also went into soakways however, He had been paying the local water authority for discharge of rainwater into the sewage. He got quite a reasonable refund for the 8 years he had lived in the property.
SO
you may want to look into that aspect