Principles of English

I rarely brag about my 10 years trying to instill the principles of English into the unwilling and malodorous youth of Ibiza. My only success was that they could all mimic my most oft used expression - SHURRRRRRUP - in a perfect St. Helens accent.

However as this photo shows, the reward we devised for getting the past perfect continuous tense wrong also helped on the personal hygiene front.

Now, complete this sentence:

I had been waiting for 3 hours when.......................