Hi Ed
I was merely trying to find a way to always enable the challenge to see how it would appear to end users should they get challenged, and also ensure that successfully passing a challenge allowed the site to continue it's operation. However it seems there isn't such a mechanism in the api. I did however manage to force it by using incognito tab in chrome (or private tab in firefox).
Strangely enough in my QA environment, we've been seeing the challenge almost everytime, while my dev environment I rarely see it), tho we suspect it may be because our QA envronment is more locked down and also not exposed to 'real' traffic, meaning the recaptcha (ie google) has little to no information on the site, so reacts more strictly; again however this is just conjecture as I haven't found much in-depth information on the recaptcha operation to confirm this!
I think the normal mode should be what you see in your QA, ie rarely being challenged.
regards
Gavin