To be more exact, there are various pension rates even for those already in the final salary scheme. The newer you are the lower the rate; fact of trying to reduce costs.
A flexi cannot be offered if it affects the pilots subsequent duties or those of another pilot
Not true. You can offer ANY day off for flexi, whether they use or not is a different matter, however, it is not uncommon for you to be asked to do a flexi day which has a knock on effect, i.e. like them having to take you off your next duty to remain legal (say 7 in a row).
You can offer leave back (all of it if you want) and they MAY need you so badly that they use you on all of it! It all comes down to how much cocking up has occured, sickness rates, uncrewed flights etc etc. Their view (as with most things) is to firefight on the day and if this has a knock-on effect, then they will deal with this later. The way the system should work (and mostly does), is to allow them to employ fewer pilots and have a degree of flexibility available, whilst giving roster stability due to more than legal minimum numbers of days off.
PP
Edited to add;
Duty Pay Captains £2.92 per hour, F/Os £2.78 per hour. 34% taxable.
Sector Pay Captains £16.64, F/Os £11.09, S/Os £9.70
Flexi Working Captains £523.69, F/Os £358.32, S/Os £330.75 (although not sure if these are completely up to date).