Latest interesting pensions rumour is that if you leave post any 'change' to the current pension schemes, you 'may' be entitled to your gratuity, but you will not receive your pension til age 60 despite having to leave at 55. Anything accured before 'change' will be honoured, but not til you're 60.
That very question was asked at a recent Air Manning briefing whilst on a course, and they came back with the answer that categorically whatever you earn before the change date (2017 is allegedly when it will kick in) is yours - gratuity, immediate pension the works. If you serve say until 2020, you would get your gratuity and pension based on service until the change in 2017 but the final 3 years of service would be preserved until you were 60 under the new scheme.
Unless of course they were wrong or telling bare faced lies - neither or which Manning would ever do. Right? Forces Pension Society would probably be the best place to go and ask that very question though.