I can confirm the bond you give ctc is paid back tax free and your salary is dropped by the same amount. So a second officer scale is £38k, if you went through ctc your salary will be £29k. That goes through the taxman then the bond is added on after.
It's a substantial tax saving when you move to the SFO scale. My bond repayment is based on the £60k bond and am about 350 a month better off as a result. Over 7 years thats £29,400 more in your pocket than the government.