What flyermanES said. Also, if there is only a short time between hitting Silver and flying again, I would recommend a call to BAEC to get the status 'refreshed' in the booking as even though ba.com shows you as Silver, the reservation may still have you as Blue. This happened to me on my promotion from Silver to Gold, about 6 days after the status changed on the website.

One slightly OT question. How many miles do you have to fly to be the World's most frequent flyer by miles?