We're effectively contracted for the full 900 hours on all fleets. You will find on both the Triple and the 400 that you will do around 800 hours per year before any over time. If you do any extra above your rostered hours, you will get paid either your 'hourly rate' (an agreed figure of about £30 per credit hour ish as a year 1) if you volunteer for extra hours or one and a half times that if you are drafted. I can't speak for the 400, but on the Triple, there's unlikely to be enough space in the month for them to fit any extra work into. The Triple tends to do shorter trips overall.
For both fleets, expect about 14 days off per 31 day month, plus leave. Expect £2800 ish per full flying month after deductions at year 1.