Drainage, drainage and drainage

I've read Part H back and forth, and I'm still not sure.
I'm converting an old chapel. It's got combined foul & rainwater drainage. Pre-existing is a 1.1m deep manhole, 2m in front of the building, followed by a 5m run to connect to the public sewer
Also pre-existing is 4 pipes come into the manhole, 1 pipe each for rainwater from each side of the building (at half the depth of the manhole), and 2 pipes originally connected to ground floor toilets at or near the front of the building (1 is still connected to a toilet, one to a sink) right at the bottom of the manhole. All are working correctly.
The only soil vent pipe was a branch off one of rainwater pipes, unfortunately part of this pipe has collapsed.
Now, ideally, I'd like to route the soil stack internally up to these existing ground floor toilet connections.
My questions:
1. For the upstairs bathroom, I'd need to have a horizontal soil branch pipe 4m long (well actually the prescribed 18mm/metre slope) from the loo, followed by 4m vertical drop, followed by a 1.5m horizontal section up to the existing point were the soil pipe goes underground. (no offsets necessary in horizontal pipework).
Is that layout likely to be acceptable to BC and likely to be trouble free?
2. Waste water from the kitchen is a bit of a problem as the kitchen needs to be at the back of the building.
a) One solution is a 5.5m horizontal pipe run up to the same point the upstairs bathroom goes underground - that concerns me as a long horizontal kitchen sink sounds likely to clog.
b) Another solution is drop the upstairs bathroom soil stack closer to the kitchen, resulting in a short upstairs branch pipe, 4m vertical section, and a 5.5m horizontal section (of 100mm pipe) before it goes underground.
In this solution the kitchen would connect close to the bottom of the 4m section. I think the advantage would be the ground floor horizontal section would clear better because of upstairs loo flushing.
The disadvantage would be if the stack blocked at this ground floor bend and backed up - it's close to the kitchen branch - yuck. There's a Part H rule about no connections within 750mm of a stack offset - I guess for exactly that reason. I can however just meet that rule with a raised area in the kitchen.
So - my question is - does solution 2a or 2b sound better?
3. A simple one this time. It would be convenient to repair that open vent pipe to one of the rainwater branches - and for the internal stack just to have an air admittance valve within the bathroom (above spillover levels). I can't be sure if that complies with Part H - though I don't see why not - whadyathink?
4. There will also be a ground floor toilet with a similar offset in the soil pipe, shorter though, so I don't see any problem there. However it's a 8m from the kitchen so I don't think it can be used to solve the problem in 2b. Any ideas here?
5. HepVO - reading their docs seems to suggest use them and no AAV's necessary - correct?
I'm also thinking that a HepVO may better protect the kitchen against backups in pipework?
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On Fri, 31 Aug 2007 12:36:25 -0000, a particular chimpanzee,

2a sounds better. If your soil stack is internal, you want to make as least likely to block as possible. The kitchen sink branch would probably be easier to clear. Your kitchen sink waste will need an air admittance valve/trap over 4m.

Rainwater pipes should NOT be used to ventilate a drainage system, unless they meet the criteria for an open stack (900mm above or 3m horizontally from any opening into a building).
Durgo say that their valve can be used without an open stack on a system serving up to 5 stacks. The old rule of thumb used to be that there needed to be an open stack at the head of any drain where Durgos were used, and the AD does say, "where there is no open ventilation on a drainage system or through connected drains, alternative arrangements to relieve positive pressures should be considered". Your rainwater gullies should be trapped, so these don't count.

Well, they are a form of air admittance valve (they let air into the pipe but not out), so they are subject to the same rules as AAVs.

OTOH, poo coming up into the kitchen sink is a good indicator that you need to get your rods out, NOW!
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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.
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