Check out
www.ppjn.com for pay scales.
You should take home around 2800 a month with no overtime as a year 1 F/O. as you correctly stated flexi (day off payments of around £360 for F/O £500 for Capt) are EXTRA AND NOT GUARANTEED. That said most pilots will do at least 5 a year at the present some a lot more. I try and average 2 a month and take home around £3300.
Also with those hours have you considered direct entry command?
BB