Tax issues are in this thread. If you're in the UK, then you pay the normal tax and NI like any other PAYE person in the UK. Other places pay tax in Portugal (20%) and NI in the UK.
The other questions you (ix_touring) can be found in the thread I've highlighted.
If you can't find it.
Overtime. Not in the classic sense, but you can work extra days. Max 6 per year. (no, I don't remember the remuneration)
Hours flown have reduced over the last couple of years. Probably because there's a lot less positioning flights now. You won't do the same hours that a lo-co new hire will do.
Standby. A bit more than used to be. I just put it down to training and I am not a trainer...
Do search the NetJets Europe website as well as OAA for the course details.