The only tickets you can get for Sydney in the new year are restricted view or day 5,
You won't get those, I suspect the Aussies will all want them
So... We have to get all of Aus out then bat to bear their score by the end if the day? Or not?
Yes and no...
Option 1) neither side bowls the other out and the match is a draw. As England have already won two matches in the five match series, and if the series is drawn, the holders retain the Ashes, England would retain the Ashes
Option 2) Australia declare or are bowled out, then manage to bowl England out by close of play. Australia win the match and have a sniff of winning the Ashes if they then win at Durham and The Oval
Option 3) Australia declare or are bowled out, then England manage to get more than Australia's lead. England win the match and retain the Ashes.
Basically, to win a match you have to take 20 wickets (or the other team declares, forfeiting however many wickets were left in that innings). If noone's taken 20 wickets in five days, then it's a draw, no matter how on top one of those sides may have been. The reason a captain declares is because he considers his side to have amassed an unassailable lead, and therefore he should be using the time to try to take wickets. This can backfire, as it did against England at Adelaide in 2006, and might just do so to Australia today if we're very lucky. If it did, that would be option 3