As reported by R. Elsey, 10/20/01:

An urban legend you may not have heard about yet is doing the rounds in the United Kingdom. I've had this about ten times in the last week, and it seems that it is taking a lot of people in. It almost took me in the first time, because the guy who told it to me swore blind that it had actually happened to him, and I guess I'm just as nervous as the next person. The legend goes something like this:

A friend of a friend performs an act of kindness for an unknown Muslim. Typically this is returning a lost wallet or giving him a couple of pounds to make up the money for a small purchase. In return for this kindness, the Muslim warns the friend not to be in a particular city on a particular day, or range of dates. The implication is that there is going to be a terrorist event on that date. I have heard this with London, Birmingham, Stoke, the London Underground and Corby mentioned as the place and with dates ranging from about two weeks ago to mid-November.

Apparently this legend dates back to the days of Irish terrorism on the UK mainland, only then it was an Irishman at a petrol station who gave the good advice.