Jimlad is correct. The deal is 21 months if you VER, 15 months if you are CERed. The 21 months VER will only be paid to those that have been in the longest with the 1 month/years service controlling it.
My establishment has just released over half our workforce, most left on the old terms. The rest of us go in a couple of years on the new terms, (the vast majority will only take the 12 months as it will be CER although under the new terms the employer MUST offer individuals the choise of taking either VER or CER).
The obvious impact on the choise is when insurances are involved. If you VER companies will obviously opt not to pay out as you left by choise. Job seekers will not be paid, apart from getting 6 months on a Contribution base.
Just to let you have an idea of the differences Jim talks about, an SAC tech with 3 years service gets at least £2000 more than I am getting with over 25 years in the RAF and 10 years with the MOD as an engineer. I chose to come into MOD so that was my own doing but there is an overwhelming idea that we are all fat cats. The real fact is many of us are doing the exact work some of you guys are doing for way less pay and pensions, (and that includes working in Afghanistan).