Your figures are for 2006; mine for 2007 show £11343 for a 21 yr SSgt.
I don't think that this will be index-linked until the soldier reaches 60.
As far as I know, he would then receive a further pension based on his disability and certain other factors, but I am at the limit of my knowledge.
I would ask why this SNCO is being discharged rather than be allowed to serve on to 22 yrs? There may be very good reasons in that he could be incapacitated or that such action may affect other grants/benefits, but I would have thought that it would have been in the interests of the soldier to allow him to complete a full career and take full advantage of the resettlement package.
VMT Litotes, agreed, it does sound strange discharging him at the 21 year point, 20% disability is the minimum assesment to be eligiable for a Medical Pension, anything below would attract a Lump Sum only.