This seems to be official MOD statement on this issue:
Guaranteed Minimum Pension (GMP) - Error In Calculation
The Service Personnel and Veterans Agency (SPVA) is sorry to have to inform Veteran Customers that an error has been identified in respect of a small proportion of Armed Forces Pension Scheme 75 (AFPS 75) pension payments. This error is in respect of the application of the Guaranteed Minimum Pension (GMP) element of the pensions of some 6,000 members of AFPS 75.
The error only affects 2% of the AFPS 75 pensioner community. Armed Forces Compensation Scheme (AFCS), AFPS 05 and War Pensions Scheme payments are not affected.
Usually, under Government Accounting rules, SPVA would seek repayment of any amount overpaid in an occupational pension scheme such as AFPS. However, in this case, SPVA are happy to inform you that GMP overpayments relating to this issue will not be recovered.
This error affects pensions administered by a number of government departments. The vast bulk of AFPS monthly payments, some 360,000, are unaffected. The error came to light some months ago and a full audit has since been undertaken to identify the scheme members involved along with the cause and effect. The issue affects pensions paid by a number of government departments including the Ministry of Defence. The process for ensuring that pension records include the necessary National Insurance contribution information has not worked effectively in all cases.
Full details here.