Only what I've posted. And it's not just BZDs. Alcohol and the synthetic opioid
dextromethorphan which is in some cold and flu remedies can too (it is also problematic because of serotonin syndrome, and probably doesn't actually work).
It may well be the diazepam, but also be aware that the risk of antidepressants working may *decrease each time they are stopped and restarted and this can apply not only to the meds previously taken, but potentially all of them, although, from what I've observed a med from a different class will often work.
* Two studies,
Amsterdam JD, 2016 and
Amsterdam, 2009 found the likelihood of antidepressants working drops by between 19-25% at each restart (see also:
Amsterdam JD, 2009;
Leykin Y, 2007).
The brain is a very resilient organ and many of the parts involved in anxiety/depression and med function such as receptors and transporters, even entire neurons in some brain areas, are not permanent structures so it is very difficult to permanently "screw" it.
So give antidepressants another shot without a BZD. Low dose mirtazapine is a *good alternative sedative to BZDs for controlling the initial anxiety spike when starting antidepressants, or following dose increases. It might also speed up their kick-in. The main mirtazapine issue is sedation which, unlike the BZDs, tends to continue for as long as it's taken.
* the antihistamine hydroxyzine (Vistaril) may be even better, but I understand NHS GPs probably can't/won't prescribe it.