psuelight,
The £42841 is your gross basic pay (before tax). On top of that you will get flight pay (£9 per flying hour) and Time Away From Base pay (£2.64 per hour away from base). Expect around £1200 per month net for a full month's work on the 777.
In practice, your allowances are about the same size as your monthly tax bill therefore your take home will be about the size of your gross basic (give or take). So expect to see about £3600 per month in your bank account.
HF