I only noticed this one yesterday and thought it was unlikely for us as we've done so few letterboxes. Then I found that we'd accidentally achieved it when finishing off RubyShoos Blockbuster Bingo Challenge. On 6/4/2012 we did three letterboxes from the BBC series (BBC20 - GC26KKF, BBC21 - GC391X8 & BBC26 - GC26KKX).
Qualifying caches on my webpage.
Earlier today we'd picked up some challenges around Bluewater and then moved on to here. First though was lunch. A squirrel sat watching us while we ate - just a few feet away
An easy find - though its a bit nettley at the moment.