I'm not questioning as I haven't investigated further yet, I just asking about the vent. I'm just hoping it's a cracked mortar issue.
but....
The gases should be escaping from the vent surely ?
If you want to attempt to get to the bottom of the issue, you need to accept there will some effort involved. There is plenty of room, you may need to lift some of those paving slabs to do a full investigation, unfortunately you cant make an omelette without breaking eggs.
Alternatively, if there is access available to the drain taking the stack from a nearby Manhole, get someone with a CCTV camera to survey the pipework and report on tis condition.
