How about
Dear MP
I read with great interest the fact that the S of S for Defence is paid his salary by the MOD, despite only working two days a week on Defence, the rest of the week being employed as S of S for Scotland. On having the matter raised in parliament The PM stated his Confidence in Des Browne saying that 'he was a hard working, committed individual, who could do two jobs at once'.
Well Mr MP, i have a proposal! As precedence has clearly been set, I suggest that all members of the armed forces should be allowed the same option, that is to don their uniform for two days a week, whilst being paid a 100% salary by the MOD. The other three days of the week, they could find some other gainful employment of their choosing. Just like Mr Browne, the members of our armed forces are also hard working, committed and able to do two jobs at once (in fact at the moment, most of them are doing 4 or 5!)
As Mr Browne refuses to step down as S of S for Defence in favour of a full time appointment I believe the above proposal to be entirely fair and reasonable. I am still bemused as to why we have a full time Culture Minister, but a part-Time Defence Minister.