Rotate:
I think it would all depend on what fleet you are on and how many hours you have.
For a Multies pilot with at least 1500 hrs on type and having had a look at various airlines and the packages they are offering, I would suggest a figure of at least £60 000 after tax for 5 years extra service.
This would bring the pay to a similar level and go in some way of making up for the pay cut over the last 5 years, AFPS 15, NEM, and the BS of day to day service life. At the end of the day, it will cost the MoD more than £60 000 to train a baby pilot to the same level.
I suspect though that there is no money in the pot for an FRI. Also the MoD prefers the stick over the carrot method of retention, and I would not be surprised to see an increase in the RoS on initial conversion to type. Or dare say it, an increase for PVR timelines.