From the Post Office website:
"Q: Can I withdraw my money early?
A: Before opening a Growth Bond, you should be sure that you will be able to leave your deposit untouched for the whole 1, 2 or 3 year term. However, if you do need to withdraw all of your money before the Bond matures, then a Bond Breakage Charge will be applied. This will be calculated and advised to you at the time your request is made and will be deducted from your capital, meaning you could get back less than you invested. Partial withdrawals are not allowed. If you require more information about breakage fees then please contact our Customer Services on 0845 602 3409 and they will be able to provide you with full Terms & Conditions"
Chris