Excellent timing, and photo! Did you see the rollers being fed? The presenter throws a locust up into the air and one of the rollers swoops down and takes it. Impressive, but tricky to photograph.
I believe they're always fed live locusts rather than crickets, and I think that's what this guy has got.
I know they do a feed every Tuesday just after 4pm, and there's supposed to be another after 3pm on Thursdays, but I haven't seen that one first-hand.
There are probably enrichment live feeds at random times throughout the week, so you could, and did get lucky! They're likely to be in the afternoons when the people in that area of the zoo have thinned out a little. They put meal worms on the rocks close to the path and that's probably best done without thousands of screaming kids standing around.
