Thanks for your reply. Hmm - in the absence of earlier replies I'd
just convinced myself 2b was better - I'll chew on it some more.

As far as the ground floor offset goes - Part H recommends a large
radius bend at the start of any offset - I can't see any reason why I
can't fit a rest bend intended for underground drainage.

It just occurred to me that the solution to the kitchen connection is
to go for 2b but connect it into the top of the ground floor
horizontal section rather than the lower part of the vertical section
- that should ensure good clearance form upstairs loo flushing and
being after the rest bend mentioned above - less likely to be affected
by any blockage.

You're correct the rainwater gullies are trapped. The old soil vent
pipe came off a branch between the rainwater gully and the inspection
pit - so it did the job of positive pressure relief.

The building is tall, 5.5m to the gutters, 9.5m to the roof apex.
Given that there's no openable windows within 3m (on that building
face) of the SVP, how tall does the SVP need to be?

Not taking it above gutter height, in fact taking it to just above the
height of the fixed lights (3.9m) would be good aesthetically.