When it comes to deep cynicism in relation to military interventions in the middle east I think you'll find it’s the efforts of a certain Mr Blair that have had the deepest and most lasting impact on the attitude of the British public.Not quite so; the evidence was provided by the first responders. There was Russian-backed social media campaign to paint them as biased.
I'm terribly disappointed that the constant efforts of our non-democratic friends around the world have moved people from scepticism about claims (about pretty much anything) all the way over into deep cynicism and a feeling that *no-one* can be trusted. Looks like their tactics are working.
Vin
You're right about the evidence though, I remembered things incorrectly. There are uncertainties about the samples though. The OPCW report (which can be seen here) says of the samples (paragraph 3.66) "the entire chain of custody could not be categorically verified" and (at paragraph 3.45) that samples were received "depending on the source, between 1 week and 2 months after the incident". So the samples may or may not be accounted for during the time they were in an area controlled by an Islamist terror group who would have an obvious motive to discredit Assad. In summary, there is no conclusive evidence.
It's also not just the Russians raising questions. A separate report from the UN talked about how a hazmat team turned up in the area and claimed to have detected the presence of sarin using a device that can't detect sarin and referred to inappropriate and unprofessional methods used by first responders and those collecting samples. So there are plenty of reasons not to be certain about this.
Having said all that the question of whether it's really Assad using these weapons is almost a side issue for me. For the sake of argument let's say it's all true and Assad is using these chemical weapons. Given the significant number of Islamist terrorists in the area and our feeble record when it comes to these interventions why on earth would anyone believe that a military intervention would have a positive outcome? Chances are it'll either lead to either another Iraq or another Libya.