How should i lay out a Job CV? How many years should i go back etc?

Answer 1

Take a look at www.cvtips.com and read the info there.

Answer 2

You need to account for your activities since you left school (i.e. since GCSE's).

If you don't it will be assumed that you have something to hide (like a year in jail).

If you took a 'Gap Year', I suggest you consider how to 'talk it up' a bit ('I worked with refugees in Somalia' is acceptable. 'I spent a year in Indonesia out of my head' is not :-) ).