We have helped in the past with invigilation but reeeeeeeeeally hope we never get asked again! At the time we had 2 more techs than we do now, and all other support staff were helping out too so we didn't feel we had much choice.
I have very occasionally been asked to supervise a couple of kids who missed the first exam for some reason - the most I have had is 3 at a time and to be totally honest I enjoy the downtime. I am allowed to take my laptop with me.
Support staff got stuffed with this one a couple of years ago at my school. None of us wanted to do it and we fought really hard against it including using our unions, but the head teacher and governors over-ruled us. They put it in writing and gave us the required notice of it being a change in our contracts so that they could enforce it.
I absolutely hate doing it and avoid it as much as I can, but that only goes so far before it starts negatively impacting my colleagues who also don't want to be doing it anyway. The school has been really struggling to employ invigiliators so the support staff have to shoulder a lot of the burden.
Not sure how it will work going forward as the school has now employed some invigilators but previously teachers did the invigilation in half hour slots on a rota. We helped out where necessary, but specified which slots we would be available for otherwise we were given lunch or break which interfered with setting up/clearing labs