It so much more important to take this stuff seriously if you only do it occasionally and for fun/recreation! Suffocation is the least of your worries, both H2S and CO are poisonous. And you don't have a team standing by with with air tanks. So response time is likely to be closer to an hour, on the off chance that there's even anyone to call for help. Think about the kind of brain damage you're liable to suffer if you do survive such an ordeal.

A couple of gas meters (the reason to have two, in case it's no obvious, is to try and make sure at least one sounds an alarm in case of faulty equipment etc) - and an "active" monitor to someone outside (aka: a rope) is generally as good as you'll get. And a lot better than what most people do.

Don't become the next anecdote at a search and rescue seminar.

I... umm... yeah. Those are all extremely good points. The idea of surviving with major brain damage is almost more scary than thinking I wouldn't come out at all. It also goes towards explaining why when I ask my friends for advice who do have industry experience, they're not willing to do more than tell me I'm an idiot and that if I want to do it safely then I just shouldn't do it.

I feel I should point out that I do try to be mindful not to become a statistic or anecdote. Apart from the obvious reasons, in 2015 it's hard enough to get away with having unsupervised, unstructured and unlitigated fun. If I do myself in through stupidity whilst exploring, climbing or gliding etc, then apart from anything else it's going to make it that much more difficult for anyone who comes after me. Imposing more restrictions or difficulty on a hobby I love is the last thing I want to be remembered for.

H2S, CO and O2 seem like the obvious gases to be mindful of; is there anything else you'd suggest looking for in an air monitor pr being especially wary of? Massive thanks for your time and knowledge :)

I suppose there are other hydrocarbons to worry about (both naturally occurring from compost, and from leaking gas pipes/old tanks). Not so much because of suffocation (that's why you have the O2 gauge) but from the not blow up/be burned alive part. I suppose it depends a bit where you go exploring - AFAIK old coal mines can be dangerous wrt fires/explosions. It's a good reason to avoid open flame/non ex certified electronics, if you have reason to suspect an explosive atmosphere. Fires can be dangerous just by binding oxygen - in addition to the more obvious danger.

I think the main thing is to realize that enclosed spaces can be really dangerous. Farmers die in grain/grass silos every year. And people die from CO poisoning from using coal as heat-sources indoors, or just from theoretically sound, but flawed fire based heating solutions (eg: paraffin/diesel/oil heaters).