Stub stack to be reduced in height

[Martin Gaffa]

hi I have so many different answers on this. Upstairs ensuite toilet has a stub stack abou 20 inches higher than the toilet cistern in the corner. It not vented through to the ceiling in the bathroom it's half way up and been tiled. The soilstack goes straight down inside the house via false wall in the hall. Can the stub stack be reduced so it's still above the toilet but a couple of inches below work surface so it will have a neat finish left to right worksurface and stub stack hidden in side just below worktop. I know guide lines. But some people say it's a long way to get blocked to come up that far. In the bathroom is a shower and a sink. Fitting a Durgo valve couple of inches below the sink overflow but higher than the toilet is this something that can be done as so many have different answers on this ?. Please advise.