There are older properties that have combined surface and foul water drainage systems so my property is apparently not contravening any regs.
You seem to be missing the point here. Combined surface water and foul water drains can be OK. The system is airtight. The underground pipes connecting it to the main sewer are plastic, or glazed, the joints are sealed and at every entry point a water filled trap keeps the system airtight. This prevents the escape of vermin, sewage and foul air from the sewer.
But you are not talking about 'Surface Water' (which is water from a roof or paved area) or 'Foul Water', you are talking about a 'Land Drain'
To this airtight system you are proposing to add a 'Land Drain' with porous, open pipes and no traps to prevent the escape of air, vermin, or sewage from the sewer.
If the gradient of the land drain was insufficient, or in times of heavy usage / heavy rain, or in the event of a blockage Sewage would flow down the land drain pipes and soak out of it into your garden.
Your neighbours system may have worked for 10 years, but, how does he know that the wet soil below the surface is because of earlier rain, or from sewage leaking out from his porous land drain into his garden.
If you do go ahead and do this, you are turning an airtight sewage system into an open drain, the sort of thing that causes the heath problems in 3rd world countries. It is a health hazard and that's why it's illegal.