According to ADVFN it was an N trade, meaning an unprotected portfolio or a fully disclosed portfolio transaction, whatever that is ?? don't know whether a buy or a sell though. A hell of a trade, could it be a bid coming?
I believe that N trades are something to do with institutions and taxation - ie they are usually very large trades, and lots take place before the end of the tax year. I think they're a bit like a roll over, and therefore they are unlikely to have too much of an effect on the price.