Has Silverstone ever been off the F-1 calendar since the first modern-era race was held there in 1950?

Yes , the race has been held at Aintree and Brands Hatch on several occassions .

F1 races were held at Silverstone the following years 1950-1956, 1958, 1960, every other year 1963-1987, 1988-present.

Source: http://en.wikipedia.org/wiki/Silverstone_Circuit BTW, you can find quite a bit of info quite readily through http://en.wikipedia.org/wiki/Formula_One and follow links to various years, tracks, race reports, drivers, manufacturers, circuits, race reports, etc. anytime you have a general question about F1. I only mention this because of the frequency of general questions that a person could likely find an answer to in the same amount of time it takes to make a post and certainly quicker than it often takes for others to respond to your question. I do want to make it clear I am not critical of you asking a question, it just seems like many people are unaware of the info that is readily available. Cheers!

Thanks for the info...will check it out.