Mark. Welcome!
Yes GATSOs flash twice - they record 2 seperate photographs over a fixed time interval so that your speed can be manually calculated from the images (that's what the callibration lines on the road are for). Its been suggested that they sometimes flash once if they are faulty or have no film in them.
The ACPO guidelines state that the prosecution threshold is 10% +2 over the posted limit, so in a 30 zone you should be safe at 35 or below (although I believe there are some areas that practice zero tolerance?).
If you're the registered keeper of the car, you should receive a notice of intended prosecution within 14 days. If you're not the RK then they are allowed a longer timeframe to track you down. If you don't see anything within 14 days you're safe, although I would suggest that you're safe anyway from what you've discribed.
For more information, and if you DO reveice a NIP head over to
www.pepipoo.com for advice on what to do next.