Ok so we had 740 Pounds for London for 3 nights.
196 Pounds came out of that for accomodation.
100 Pounds for the coach from Stansted to Gatwick airport + 8.20 for the shuttle bus to our Hotel (Copthorne Effingham Gatwick Hotel).
We purchased some Oyster cards (like Go cards) for 34 pounds (17 each kids free). We did not end up needing these, so if anyone wants to buy them off us let us know, they have 15 pounds credit on both of them and can be used on the tube, bus and ferry in the city.
We found that the little cafes at the airport and train stations were great for breakfast alot cheaper than getting it at the hotel. Sausage on a roll 1 pound and coffee 1 pound, so brekkie for 2 pounds is pretty good.
You can also get fruit and veg packs from Marks and Spencers for a great price from the stations as well. 2 for 1 fruit salads 5 pounds. 3 quiches 5 pounds.
BRING ADAPTORS!!! I suggest getting a worldwide one and packing a power board so that you can charge your phone, computer and the kids nintendos at the same time with only one adaptor. (Thanks Dad for that suggestion).
When we went into the city the train cost 22pounds for the day for the 4 of us from 930am(off peak) they don't really have afternoon off peak times just ask at the station when you purchase the tickets.
We also decided to purchase the 48 hour hop on hop off tour bus tickets 87 pounds for all 4 of us (these were great) you get an idea of where places are and you see all the tourist places. We used The Big Bus Company, you will see people at bus stops selling these tickets, buy your London Eye tickets through them too (trust me it will save you time).
London eye was 54pounds a family of 4.
A pub meal in the city will cost about 40pounds but I was told by a very nice taxi driver that you can get cheap pub meals at the Wetherspoon pubs (they are everywhere, just google).
So that is London done we ended up spending 720.50Pounds. $1250 AUD (remember we had to buy everything here, no self catering facilities).
Oh and buy Harrods bags from the airport they are around 30pounds for ones that look like nice handbags and 17pounds for the shopping totes.