But if you do opt out, am I right in thinking that would effectively be a double pay cut?
MOD would be unlikely to agree to alternative arrangements in lieu of your pension contributions as you might be able to negotiate in a civilian job? Therefore you'd suffer an immediate cut equivalent to the amount your salary is abated by to take into account the 'non-contributory' nature of the pension, and then a longer term cut as you would effectively lose out on the compounded growth of that 6% (?) per annum.
Or could you reclaim your abated salary and park it in an ISA or other investment vehicle?