John Carlyle-Clarke wrote:
We have a septic tank in the garden and an underground soilpipe that
feeds into it. The downstairs toilet drops into it via 110mm plastic
pipe, which has a vent up through the roof. The downstairs bath and
sink use standard 2" waste which feed into an open ground level drain
outside.

We added later an upstairs shower room with toilet and sink, which has a
smaller (2" maybe) air admittance valve in a cupboard behind it.

We are now redoing the downstairs bathroom and want to remove the vent
and replace it with an air valve inside the WC wall unit.

(1) Is it OK not to have an outside vent in the system?

No.

Durgos work as anti=siphoning systems. But a positive pressure relief
system is also needed. There must be at least one open vent somewhere.

However you can simply run a pipe into the loft space and take it where
you want..if the loft is cold..I THINK its OK just to exhaust there. Or
run a pipe to a gable.

DO be careful though..horizontal pipes with sags in them collect
condensation in cold weather, and thus dribbles down th vent out flow..
best is to either have an uphill run to the exit, so water runs down to
th drain, or an 'arch' so that at let the water run out as fast as its
condensed, and doesn't collect. DAMHIKT.