would it be unreasonable of them to expect that the employer had warned them of the prescence/potential of stinging insects in the work place
Seriously?
Which workplace in the UK would not have this potential? Unless you work in a specialist sealed environment everywhere would potentially have wasps.
health questionaire for employees there is a reasonable chance that someone may suffer from Anaphylaxis.
That would be reasonable and to make the first aiders on site aware. I assume that if you suffer from the condition you would carry your own “epi pen” or whatever.
subsequent full investigation
As HSE ask, what are you actually investigating?
If someone was stung and it turned out they suffered from Anaphylaxis then that would need an investigation but not the actual sting. You would need to be thinking about your induction procedures and certain individuals working in certain areas may require a risk assessment. e.g. Someone suffering from Anaphylaxis working next to bee hives.