Thanks, I just remembered I have a Xenon strobe at work I don't use any more so I'll give that a try. Hope the tube hasn't gone, they're not cheap (assuming you can still get them).
Out of interest, around 1 in 100 people have epilepsy, and of those around 3 in 100 people with epilepsy have seizures triggered by flickering light. That means the likelihood of someone in the theatre having photosensitive epilepsy is around 3 in 10,000, but if you have 100 people in the venue then it's a 1 in 33 risk which is why it's worth issuing a warning.
Most commonly the trigger is between 16-25Hz but can be anywhere from 3-60Hz. We use 30Hz in the Eye Dept and I have never triggered a seizure (yet).