BT, DT.
Later I went to my local branch of a Big Computer Retailing Chain where I bought the thing in the first place.
"You should have come here sooner," said the help guy. "We could have loaded all the updates at once from here," he said, pointng to a nearby fearsomely-large computer. "We do it for all new computers leaving the store, though they usually only need a couple."
Well, now I know...