Illegal connection to soil pipe?

My aunt wants to have a washbasin installed in a small WC room on the first floor of her flat and has called a plumber in to give her a price for the job. The existing pipe for the loo runs through the exterior wall and connects to (what the plumber calls) a branch soil pipe that runs horizontally on the exterior wall to the main soil stack. This branch soil pipe is only about a metre long and also has another WC from the upstairs flat connected to it also (though this connection is made on the inside of the wall).

The plumber reckons that there is a real problem with the way the pipework is arranged in that my aunt’s kitchen waste pipe runs into the end of this branch soil pipe. This waste section also runs and is connected on the inside of the building. The plumber says that this arrangement is completely in breach of the building regulations and if there was to be a blockage in the loo there could be serious consequences with regard to the kitchen sink. He says that the kitchen waste should have a separate connection to the soil stack. He says that my aunt should take it up with the council who are the freeholders of the building. She is currently a leaseholder but was a tenant originally. This kitchen waste connection was likely to have been made in the late sixties before she was a tenant.

Any ideas as to what we should do here, please? If this set up is so wrong who is liable for the rectification? Thanks very much for any advice.

Whether it breaches regs is a matter for BCO to comment on.

I would be inclined to do as your plumber suggests and fit a new boss to the stack and replumb your kitchen waste to it.

Does sound feasible that you could have a blockage problem in the future and you don't really want **** coming back into your sink

Materials probably cost £10 plus a couple of hrs labour