If I get any blockage in the soil run in the loft then it's backing up in the shower. Again if the shower trap dries out we're letting in fumes straight from the cess pool rather than one longish waste pipe that's open to the air where it exits above the grill.
25 x 450 SDS bit - cheap before it was in the clearance section!
https://www.screwfix.com/p/titan-sds-plus-drill-bit-25-x-450mm/30352#product_additional_details_container
Bastards! Sitting here with a pen and paper thinking how this place could be redone. Got as far as putting North on the paper! However one of the 11 year old sleepover lot wants to be an architect, maybe I'll ask her!
The shower/basin waste run as is (in white), just ends above a grill over the drainage gulley as per the sketch on p77 so no gas ingress as such but maybe a little "stinky waste pipe" smell yes. SWMBO really not keen on dropping shower etc into the soil pipe so I reckon it''l be my long run idea but with a universal solvent weld AAV like this and then run the wc overflow separate and out the eaves.
Thinking further on this...the current run in white, compression fittings has never had a issue with the shower or basin discharging and there's no AAV in the run. Is it maybe that the upstairs ensuite overflow thst drops into this pipe is acting to allow air into the run? In the last pic above it's the small white L shaped run appearing from the insulation halfway up the wall.
Could you maybe drape a full length tarp along the wall(s) where the upstand is, just before heavy rain perhaps tucked up under the gutter to negate water running down the walls and behind the flashing etc?
Behave! Telephone & electric poles along with DG windows aren't original nor is plastic guttering, just fit the socket as suggested. Best take down and TV aerials too for originality! The linked drill from Screwfix, is that long enough?