Working in London is a good laugh, I do miss all the fun pub outings, otherwise known as "weeknights".
The non-overtime thing is a quite rubbish (just make sure you get a decent wage to begin with).
The work can be mostly regular 9am-6pm hours, as Christian states, but it really depends how long the project runs.
Hopefully it'll be OK, making up for all the commuting you'll inevitably be doing.
Look to live on the train line to Euston or Charing Cross, depending on which office you head to, then you can avoid getting onto the Tube (which is nasty)!
Would imagine you'll shack up in the glorious English countryside, visiting castles on the weekend and jetting off around Europe occasionally
- after all, one of the best reasons to go to London is that its quite easy to take excursions to the continent.
The weather will make you miss Sydney, although it doesn't actually rain as much as people will tell you.
England in spring is really beautiful too:
You won't get such ludicrous tax breaks as Australia, but the price of bananas is significantly less.
Also beer, a staple source of nourishment for Londoners, is pretty cheap.
London's sure got a lot of entertainments, but do go see England - a much friendlier place located outside the M25.
Hope it works out,