DURGO Valves Drainage and wastage systems and plumbing help, advice and answers

[Newby]

I have recently bought a house with a bathroom, 2 en-suites and a downstairs toilet. All the toilets have been fitted with DURGO valves and they are all finished off low to the ground (not above sink overflow level). There doesn't seem to be a problem with smells or flushing etc, however everywhere that I read about these systems it states that they need to be above the highest floood level (usually basin overflow). Do I need to change the height of these DURGO's? The house is approx 20 years old and the pipes have been as per throughout as far as I know.

[htg engineer]

do the sinks waste go to the soil pipe ? or do they go outside to a gully or hopper head ? if no problems with smells, flushing etc. Then the durgo valves are working and can be left alone.